rjfguitar is partly correct.
you need a special outputshaft, the TV cable, lockup wiring, and drivelines.
you do not need a new adapter as the th350 and 700r4 share the same pattern. the crossmember is also the same. you will have to redrill the frame to move the crossmember back almost 2".
the th350 is like 1 3/4" or 1 7/8" (can't remember) shorter than the 700r4. so your driveshafts will need to be lengthened/ shortened for that.
so output shaft is like $100 from AA, TV cable is like $20, lockup wiring oyu can do your self. drivelines shouldn't cost that much to have lengthened/shortened.
ok now for tranny tech, the 87 and newer 700r4 are alittle better in stock form than the older ones in stock form. after the upgrades, they are preaty much the same strength. if was you, get it rebuilt no matter what, not worth the hassle, besides you have to tear into it to install the outputshaft.