Apparently there are a couple of different pump designs, even among the SBC apps.
The pump I have, based on the BBC design, has a recessed area in the casting where the oil pump mates to the block. The oil flows into this recess, which is at the top right in the below photo:
And there is a matching cast in recess on the main cap, this is the new engine I got:
When you install a BBC or BBC-style pump on the main cap, the mismatch is a disruption to flow, but due to the cast in recess in the pump discharge area, it's not really going to contribute much to limiting flow.
The one that came out of the 1990's Vortec block was this design, and the new motor is also. The new motor pump is an M155. This is what some of the new(?) SBC pumps look like:
Any misalignment IS a restriction to flow with this design. Car craft or some other did a HP comparison between various high pressure or volume pumps, and "standard", but all Milodon. IMO those results are misleading. The Milodon pumps tested, even the "standard' are not cast like the ones GM is using. All the Milodon pumps are cast like the BBC pumps in the oil discharge/rear main cap mating area, so a mismatch with that style is going to result in nothing like it will with the above pictured stock pump.
Going back to this picture:
I laid out some dielectric grease and bolted the M155 long block oil pump back up to the block. Not the best material to use for this, but what I had. Not real clear in the photo, but you can see a portion of the semicircle of the misalignment there. This is worse than with the BBC-style pump FWIW.
Anyone that is a math major, please feel free to check my work.
I took some measurements, then used an online calc to figure out the overlapping area. The discharge passage on the M155 pump is 1/2". The center of the circles are .34" apart. The overlapping area is right about 20%. 20% restriction to me seems pretty significant. I would assume if nothing else it results in wasted energy. Obviously not a huge concern, GM didn't seem to have an issue with it. I just find it interesting. I would not be surprised if the tests that show the BBC pumps costing little in the way of HP are a result of this, but you'd have to know what the discharge side of the pumps looked like that were tested.
Also interesting, I can find little out about the M155, but it definitely appears to be the redesigned "weak" castings that are prone to break under hard use. The M99 (BBC pump designed for the SBC) is a much more substantial piece of hardware. For $45, you seem to get quite a bit compared to the $20-35 pumps.
Edit 26JUL16: ARP 230-7003 stud kit fits perfect in the rear main cap. I put a dab of grease on the end of the stud, threaded into the main cap, and it did not make contact with the bearing. Un-threaded portion of the stud bottoms out before the stud hits the bearing. Make sure to check this clearance however, bad news if the stud puts pressure on the bearing and with manufacturing tolerances and different year blocks, it could be an issue. Stud length is near perfect with the BBC-type pump, the windage tray will take a bit out of the thread length engagement however.
FWIW, the stock windage tray does not work with the BBC-style pumps. And that is why a longer stud or bolt is needed, the BBC pump is a bit taller in this area. I'm going to cut and weld this tray back together to fit. Just need to add a "step" to reach the stud. Might need to trim around the pump body a hair, but just.
Also found out that nearly all of the additional capacity of the summitracing pan is at the FRONT of the pan. I measured it all out, this pan is 3/4's of an inch deeper in the shallowest part of the pan than the stock 4qt pan. Roughly 4.25" deep vs. 3.5".
Edit 27JUL16: Summit pan doesn't clear the front two pan stud/nuts that come with the stock pan, the flanged nuts hit the pan. A fair hassle to fix, the studs have to be removed to use a punch and dimple the pan corners, but studs won't go back in with the pan on, so unbolt everything, remove pan, install studs, repeat until the nuts go on.
Made an oil pump pickup install tool out of 1" id tube of some sort I had lying around, got pickup started via eyeball, tested fit, perfect. How often does that happen?
Tried heating pump body up to about 150-200*, and freezing the pickup, didn't seem to affect the interference fit between the two, still required quite a bit of effort and certainly couldn't be installed by hand. Makes checking pan to pickup fit really easy if you cut a large hole in the side of the pan. I figured now is the time to weld in the oil temperature sender/sensor bung. Have to bolt the pan to the spare block (don't want to ruin the pan gasket with heat) and will weld it up. May force me to buy a ~$200 engine stand from summit, which is hand cranked.
, I'm quite satisfied. I'm sure it could be improved, but metal wheels with brakes, and the crank make easy work of things. Didn't try, but it probably turns the engine over with one finger on the crank, and it holds wherever you stop the crank. Rolls on the floor really nice, which is welcome. Better have some 3/8" threaded rod handy...even the longest starter bolts I had aren't long enough to reach the block. 
