Jim,
Take this for what it is--a story.
When my engine was out 1n 1999-2001, I asked the same question. There was no group like this, however.
In the interest of saving money, I opted to leave the transmission alone,
as opposed to swapping out for a rebuilt.
After all was put back there was a transmission leak.
Took it to a place here, and the mechanic comes back and says, "good news and bad news. good news is I found the leak and it's a seal that cost $5. bad news is labor is 8 hours to get the tranmission in and out. OK, there goes $600 just to fix the leak. Oops, $605.00
Fixed the leak and all was well
for about 2 months. Started making noise.
One day it just exploded. :'( The casting of the transmission case was broken. U-joints twisted. Drive shaft destroyed. :'(
Cost to repair: new tranmission $1,200; new drive shaft $500; labor $600. That was in 2001.
Total savings initially: $1,000. The cost
after the fact, of my savings: $2,900. One of the few, but large mistakes I made on the car.
I know what I learned is this: unless I personally am absolutely, positively 100% knowledgeable about the integrity of a big expensive part that cost a lot of time and effort and money to remove, and the service life remaining,
it will get replaced or rebuilt before I put it back. But hey, that's just me.