[wagonman76]

The only 4 cylinder I can think of that might work is the 2.5 iron duke. They used them in lots of older vehicles, including a handful of early 90s Lumina sedans (also w-body but earlier design). You could probably mix and match GM parts and make one fit, but I dont think any gain would be worth it, if you even get a gain at all. If youre just putzing around town then maybe, but on the highway I think it would run out of breath way too soon.

Id see what you can do with the 3.8. Great engine, butter smooth, lots of torque, and it can do good on gas if you keep your foot out of the floor. Since you have access to computer reprogramming anyway, Id really look into bolting in a numerically lower final drive if youve got the 4T60E. There are websites that list what vehicles they are available from then go pull one from the junkyard. If it doesnt do any better, than just go back to your old one.

Good luck with whatever you do. If you find something that works well drivetrain-wise, Ive got a 3800 Trans Sport van that I might try it with.