Wrench. I help when I can
The original engine in my truck is a 350 5.7 was standard in the z71 k1500. The donor was indicated by paperwork to be a GM crate engine, same format 350 5.7 from a 4x4 Tahoe.
The first generation 350/5.7l vortecs were made from 1996-98. Then slashed by GM in 99 for better injection.
Maybe the mechanic who is now pushing daisies meant that one was TBI and the other vortec. Either way, the block assembly stayed the same but I am pretty sure the ignition CKP reluctor wheel was different from one year to another. the 96 had only a three slot reluctor wheel while the 97 had a 4x or a 3x.
These weren't the best years of the 5.7l and that is why they varied so much and only stayed first gen for two years.
So as billr suggested, pull the crank sensor and count the teeth. However, if it is running and idle fine, that should imply all is good there, if not, this should be a no start instead of a stalling while in gear.
But for sure, they ARE THE SAME BLOCK.
The only thing I can really see here, is that one engine was maybe a vortec while the other was TBI. That would cause a start and run, but would be a bitch to keep running in gear if that were the case. An easy way to check that would be ECM service numbers from the donor vehicle to your Z71, but again, that is all long gone with the dead HOG loving, poker run guy.
Start there when you call the dealer, they would have that info. Alldata is quite generic and not as specific as a dealer database would be.
I hope I helped instead of confusing the issue.
My other humble opinion is that both the Z71 and the Tahoe taco both came with the sports package that had sequential vortec on board, so again, I am quite sure that if the info YOU have is correct, that the donor came from a taco 4x4, and put into your Z71, both engines should be plug and play all day long. This I know for a fact.