Gavin, there are essentially 3 types of TH350. Short and long-tailshaft TH350s and the TH350C Which is a slightly different animal and uses a locking torqur converter. I don't believe the 350Cs are as strong as the generic 350, but I've never really tested them. Btw, you can tell if it's a 350C by the presense of a 4-wire electrical connector on the left side, just above the pan flange.
Imo, the best tranny to use is the short-tailshaft, plain-jane TH350. Same length as the PG, TH400, Saginaw 3 and 4-speeds (not sure about the SM-420) and the TH200-4R. The TH700R4 is 3 inches longer...and the TH400 uses a different driveshaft yoke.
That plain vanilla 350 is a sturdy piece and will generally last a long time....of course it's possible to kill any trans...but the 350 is pretty good. The last time I saw one blown up was when a "friend" was showing another guy how to "peel out" by doing a 5000 rpm "neutral drop." I don't know of too many transmissions that would take that kind of abuse for long.
hth,
