Pete,
Mine came with a ST300. I put a TH400 in it in '77
and that wasn't too hard at all. Since then I went to the 200r4. That too was a breeze in regard to using existing and come creative salvage parts scouring, primarily for the detent cable bracket. Get one off of a later model Pontiac powered car and it will work in conjunction with your accelerator cable bracket.
The travel of that cable is ALL important in the way it will shift. As far as I know, THAT would be the same with the TH350.
I used my TH400 cross member and didn't have to modify it, but I did use a couple of spacers to properly clear the pan and cross member.
I used my shifter throughout all the changes, but the shifter cable bracket on the transmission I did get from a place (I think) called gearworks.
They have the detent plate to go into the shifter housing so that you have a stop in each gear. I did that too.
I did not use a lock-up converter, but that was by choice.
It IS a tight fit for the speedometer cable in the tunnel. I did go through at least a couple of cables and tried an adapter (corvette gear drive tachometer adapter is the same size as the fitting, but was too bulky to offer an advantage).
The transmission cooler lines will need to be messaged, and I lost a little room to change the PF24 oil filter, so I went to a PF61 (skinny) filter. I don't feel bad about the size of the filter as often as I change the oil.
You will most likely need to have the drive shaft modified and I'm not sure about the slip yoke for the transmission. The one you need however, is a very common piece. Measuring for a drive shaft is not as much of a problem as people make it out to be, however is very VERY important.
High performance Pontiac did a series of articles a couple-three years ago on it. I saved all the info that I felt necessary to keep and would be willing to share it with you should you decide to go the overdrive route.
With the 3.73 rear end, for me it was the best piece of work I've done in the 31 years I've driven the car.