My 2¢.
On the drive to Blazer Bash I got just a shade over 11mpg with the 454 and Holley Sniper. I have 37's, 5.13 gears, and OD. That is pulling 10000ft passes over I70 too, so I think that's a good solid MPG.
I weighed my crew cab while it still had the 350 and it was 7700lbs. That was on a certified Cat scale that guarantee their weights. I was shocked it was over 7000. I was guessing it would be around 6500. I'm going to weigh it again after I put the 4L80e in because it definitely weighs more than the 700r4. Not to mention the weight of the Magnum and NP205. I'm betting the addition of those 2 plus the 454 is going to put me at 8000lbs.
I have a 2008 2500HD company pickup with a 6.0L and the 6spd auto, 6L80 or 6L90 I think. I have mostly a highway commute and the best mileage I could ever baby out of it was 14.5mpg. That truck weighs about 7200lbs according to the scales at the salvage yard.
My saddle tanks are factory "baffled" because it came with TBI on the 350. I just swapped in the higher pressure ACDelco pumps, can't remember the part number now but it would be factory on a 97 Tahoe. The baffling doesn't do much under 1/2 tank on severe angles. For street driving I don't notice a problem until I get under 1/4 tank. IMO, the saddle tanks a much less forgiving for fuel slosh due to their shape. I feel like the K5/Burb tanks are less finicky based on the experience I've had with them.
Coming from a TBI system made the accessory costs of installing the Sniper a little cheaper. I only needed 2 pumps and the fittings to adapt the steel lines to -6AN. If you are comparing LS to aftermarket EFI, any fuel system requirements are going to be the same for either one so you could consider those a wash.
You can do the aftermarket EFI and hold off on the timing control for however long you'd like to spread out the costs. I sprung for the more expensive Holley Dual Sync distributor for timing control with the Sniper because I knew they would work well together and product support would be no concern. If you are looking to keep costs down,
@Bent77 figured out a TBI distributor will work with aftermarket EFI. You can get a decent all new TBI distributor for a little over $100.
I think it's likely you could do the aftermarket EFI cheaper. I think if you decided a year later, that the EFI'd 454 still wasn't cutting it you could do an LS swap and get 75% of your investment in the EFI system back. Your fuel system would already be setup for the LS setup.
The other thing that appealed to me with the Sniper is that it will support up to 600HP. So you could also pull the 454, rebuild/hop it up, and stick it back in with the EFI all ready to go.
