Transmission options and opinions
The only aftermarket transmissions out there cost big bucks and the only advantage they'd have is overdrive. At 350 hp your needs are pretty basic. Do you know a REPUTABLE transmission shop near you that knows older transmissions, especially torqueflite's? I highlighted "reputable" because there are those that aren't so don't take it into a shop without checking them out good first. A good basic rebuild with good quality parts should last for yrs. I'd replace the converter too since it has 100,000 miles on it. Most likely your 350 hp engine will like a slightly looser converter anyway. Something on the order of 2500 stall will let the engine idle better and really perk up the acceleration without affecting drivability. Installing a trans cooler will make it last a lot longer too.
If you don't know a shop that's reputable and good with torqueflite's start a thread in the general forum asking who's good with torqueflite's in western New York and I bet you'll get a lot of replies.