Thanks all, I think I got my answer (not to worry about interchange differences, if they exist)
The transmissions on these vehicles (Trailblazer/Envoy/Rainier) are easy to tear out/slap together… its not like working on a subcompact where everything is super tight. I can get a used 4L60E transmission with 120K miles in my area for $500. A certified rebuilt transmission is roughly $3000 plus $1400 install.
That means, since I am doing my own work, I can replace the unit a few times with no worries.
Heck, I can rebuild the original for about $300 and I have a backup waiting, just in case.
It’s a bit higher risk, but I’m OK with the possible outcomes.