The 2.8L is one of Chevy's worst motors ever, and I'll never use one in another truck or re-build one again.
I'm told that the 2.5L is lousy too.
Try getting a complete running 4.3L truck around the same year that's been rear ended hard (trucks involved in front end collisions might have internal engine damage)
Then take 1 part out of yours at a time, then that part from the donar truck and transfer it over, 1 piece at a time.
Good Luck, it's a lot of work, but worth it if you have the time and patience