Just what was the TH350 doing to be considered "going out" ?..
It might end up being something simple to fix--I bought more than one GM vehicle with a TH350 that was supposedly "blown",but all that was wrong is they would not upshift until you hit the redline in each gear..it was the vacuum modulator (or no vacuum going to it,or it was hooked up to a ported vacuum port, when it needs manifold vacuum)..
When the modulator isn't getting vacuum or it's diaphragm ruptures,it will be slow to upshift,usually it wont go in second until your going like 30+ mph,then it'll hang in second up to around 45-50 mph--then finally go into third..then,when you let off the gas,the transmission will feel like there is no engine braking,it will "free wheel",or feel like its in neutral--applying throttle again will make it feel like it goes back into gear..
One truck I bought would not go in third gear--it was the detent cable had stuck in the "passing gear" position,it was kinked badly behind the engine..another cable cured that issue..
Only one TH350 I had did have serious issues--one of which was "no reverse"..sometimes it would go in reverse if you were willing to wait up to 5 minutes after shifting to reverse..but it felt weak ,and it would slip if you had to back uphill..one day I couldn't get it to go in reverse any more,so I bought a known good used tranny to swap in..
One I had in a '72 K5 would refuse to go into any forward gear until the engine warmed up..reverse worked perfect instantly!..
I tried a new filter,fluid,modulator,with no difference..several guys familiar with TH350's said it was the forward clutch piston seals,they get hard after years and the piston wont apply pressure to the forward clutches until they warm up and soften and let it move..
I was told to try adding a pint of brake fluid,that did nothing,then a few other guys suggested trying various additives,I even tried a pint of Lestoil cleaner one guy swore by ,to free up sticky valve body parts--nothing changed,I couldn't move it until the engine started warming up,on winter mornings this might take a good 10 minutes--once it went in forward gears it drove fine,no slippage,and shifted normally,I could drive it 500 miles with no problem,but let it sit overnight ,it did the same thing again..
I ended up putting a SM465 4 speed manual in it--had another truck for a parts donor..had to change the driveshafts,install everything behind the engine--not a job I'd care to repeat even if I were 35 again..
Bottom line is your TH350 may not have major issues...but even if it does,they are one of the easier transmissions to rebuild I'm told..
I never "missed" not having overdrive--most of my 1/2 ton trucks had 3:08 gears and they cruised at 65 no problem,had a few with 3:73 gears,they wound up higher on the interstate,but not excessively --if they had 4:56's then I would have been wanting overdrive..
so decided to find less of a project and pulled the trigger today.
