Can I upgrade my 5.3L V8 engine ?
 

Hello everyone,
I'm not really familiar with Chevy & GMC . I specialize in Subaru WRX / STi engines built and I am long time experienced mechanic but I have no basic knowledge about GMC & Chevy engines. Long story short:
I own a 2003 GMC Savanna REGENCY Legend conversion van with 550,000 miles, 3rd transmission, engine runs strong and it does NOT burn oil , oil change every 6K miles, the last 350,000 miles my van pulls 7x14 trailer with at least 3500 LBS total weight, climbing Colorado high rockies, California High Sierras. I have to replace water pump twice and oil pump and that's it. I don't want to sell my van as it is pretty rare and exterior / interior look like 50K miles , or even less. I would like to have a little bit more power , I am open to engine swap alternatives. Can you recommend me which directions should I go? Can I drop 5.7 or 6.0 V8 w/o any major issues? Any help will be much appreciated. Thanks!

A 6.0 Ls engine with 4l80e trans will swap right in with little to no modifications depending on what year you pick up. They come out of the Yukon’s Escalades and some Silverado/ Sierra HD or 2500 models. I say little modifications because sometimes the throttle body is different. And maybe some sensors on the newer year engines. Make sure you get a computer( ECM) with wire harness or have access to reprogram your current ECM for the 6.0 values. I also recommend the trans just because you don’t want to have to replace that down the road. But if you prefer to keep the one you have the 6.0 will also bolt right up to your existing trans. Assuming it’s a 4l65e.

Thanks!
I was wondering if I ned to merge harness but if there is a way to reflash the ECU, that would be awesome. Hell, I can pick up a whole donor car from iaaor copart for 2-3K. this way i can change the rear end as well.

Forget the 3.73 if you are pulling those high altitude upgrades all the time. Your gear ratio needs a 4.10 ratio or lower rating for that kind of towing.