I purchased some stuff from these guys (links direct to the splayed merge pipes you may need)
http://www.mandrel-bends.com/catalo...splayed-2-1-merge-collectors-180/index-3.html When I got rid of all the dual exhaust junk and went back to single.
I got on their "b-stock" mailing list, and ended up buying 25 pounds of 2.5" mild steel bends for $25 plus shipping. Worked out to about 10 pieces as I recall. Some 180's, some 30's and some 45's. I needed to make the y-pipe, pretty hard to beat $25 for pre-bent stuff that I just cut and welded to fit my needs.
I made my own two into one merge...the angle coming across under the rear of the bellhousing made it appear no merge collector would have the right angle. It's kind of nasty looking, and took a fair bit of MIG wire, but it works. Plus it wasn't $60. I didn't find any "cheap" 2.5" to 3" merge collectors.
Rethink your O2 bung replacement. It doesn't need to be after the merge. Put it forward as far as you can, while still capturing all 4 cylinders on one bank. I would suggest mocking everything up and seeing where the sensor will be easiest to access but still out of the way before I welded a bung in.
Converters IMO if going aftermarket, go with the smallest body you can find, from a big name. I've heard that the aftermarket converters are nowhere near as sturdy as the OEM ones, and price difference probably indicates that's reality, however the smaller body converters just keep the converter further away from everything, and that's good. I suppose you might be able to get away with one of the round converters, but to keep it tucked up tight, the flat style high flow converters are more likely to work better.
FWIW, on the collector bolts....don't waste time here. Get as many as needed (or twice as many, for the future) that are just plated steel. Grade 5 seems easiest/cheapest to find. Anyway, just use these as the collector bolts. Tighten them up properly, then tack weld the nut in place. The flange bolts come loose when the exhaust is not hung properly, when the engine moves, when the frame twists, etc. Tack welding the nuts ensures they don't come loose. This is why GM used springs on the manifold studs to the exhaust pipes. I have poly engine and trans mounts, and the exhaust pipe mounts are as solid as they can be. I've attempted to eliminate all movement I can to this point, which SHOULD help the exhaust from loosening up over time. Use cheap hangers that allow too much movement, that WILL fatigue the collector and put stress on everything up to the cylinder head flange bolts.
I'm to the point now where I don't need or want to hear the engine. I am more concerned with being able to hear the radio or passengers. I purchased one of the Walker Quietflow SS's that are a 3" in/out replacement for the GM stock muffler on later K5's, as I couldn't find any muffler advertised to be quieter than stock. I intend to go full stainless at a later date, the mandrel bent mild steel was to see what worked and get me by for a few years, thus the stainless muffler. I ended up cutting the rear half of the exhaust off on the '87 K5 in the garage in one piece to save myself the hassle of making the over the axle bend and welding the muffler up, plus mating the stainless muffler to mild steel tubing. The '87 had a tiny 3" in/out flowmaster muffler on it when I bought it. Definitely quieter than the no-muffler setup on there before, but nowhere near as quiet as I want. The larger bodied mufflers are where it's at IMO. FWIW Walkers website is pretty good....use the online catalog to select your vehicle, and then use the part number for the muffler over on summit or wherever to get the specs. As yours is an '87, be sure to check out say, a 1991 R/V-series muffler. I believe for the K5's Walker showed a split in 1986 in the muffler design, the later in year you can go, the more likely you will be to find 3" mufflers.
Not sure how this will post, but here was my summit order for the rest of the parts to put mine together:
WLK-33278
Exhaust Clamp, Band-Style, Butt Joint, Stainless Steel, 3.000 in. Diameter, Each
(Mfr. #: 33278)
WLK-33240
Exhaust Clamp, Band-Style, Butt Joint, Stainless Steel, 2.500 in. Diameter, Each
(Mfr. #: 33240)
WLK-36516
Exhaust Hanger, Bolt-on, Steel, Natural, 10 in. Length, Each
(Mfr. #: 36516)
WLK-21054
Muffler, Quiet-Flow 3, 3 in. Inlet/3 in. Outlet, Steel, Aluminized, Chevy, GMC, Each
(Mfr. #: 21054)
Of that order, I ended up using one 3" band clamp to make the y-pipe removable (header flange bolts plus it and it drops out forward of the trans crossmember). I only needed one weld to get the rear piece I already had forward to the y-pipe, so the rest of the clamps went unused. If you don't mind drilling holes, the exhaust hanger listed is good, but I didn't want to drill, so I monkeyed around and made that hanger bolt through one of the oblong holes GM had in the frames. I re-used the hanger behind the converter, and the one at the very end of the pipe before it makes the 90* exit, as my opinion is that those keep the pipe from moving vertically. I mounted the aftermarket hanger horizontally, to keep the pipe from moving that direction. The one flaw I see with any of the aftermarket exhaust hangers is that these trucks are not set up to allow easy "hanging" of the exhaust...the only solid piece under there is the frame, and if you tuck the exhaust up like it should be, there is no way to use a vertical hanger, except using at least part of the stock hangers since they protrude off the frame. Thus why I chose to use the two existing vs. trying to make the aftermarket hanger work at weird angles.
I will tell you that the amount of warping experienced from welding up the Y-pipe was tremendous. Fit everything up, tack welded it while in the truck, then took it out and welded it all up. Went to put it back in, it had grown so much that I had to cut it in half and take a bit out. Test fit, all good, welded it back up taking more time to ensure it didn't get overly hot. Went to fit again, now the two flanges weren't square. They were close enough since I was under a time constraint.
I get a bit of exhaust leakage when it's cold, and it looked like the band clamp might be leaking a bit (even though it comes with some sort of pre-applied sealant goo to prevent that leak) but since the O2 is forward of any leaks (its in the actual header) it really doesn't matter other than being annoying that I didn't get everything sealed perfectly.
Prior to this the truck had duals from the headers back. I went with slightly shorter headers, and the pipe now goes from 2.5 (header outlet)->3->2.5->3" due to the pieces I had left around. To be absolutely honest, I cannot tell the difference before and after in terms of performance. While going with mandrel bent exhaust and not necking down the exhaust would definitely help performance, I really, really don't think that at this power level, there is any way you could feel any difference. I cut corners because this isn't a permanent solution and I had a few hours to do it. If you have the means to do it "right", do so, there is no downside. I just don't think it's necessary to worry a ton about mandrel bends and exactly the proper diameter pipe if saving money, or spending it on some other component, is in the cards.
FWIW, I used the truck continuously for 10 days on logging roads, definitely not being easy on the potholes, and it didn't loosen up at all.
