I beleive a TH350 CAN be modded to a 4x4 by installing the main output shaft from a 4x4 version also...far as I know the Powerglide is the only automatic that cant be converted to a 4x4 application,as the tail housing is part of the case and dont unbolt...(you could use a divorced T-case though!)...
The 4x4 700r4's have a thicker bell housing (K case) and they have the larger bolt holes for a cast cover and the strut rods to bolt too--a car or 2wd will usually only have a tin cover held on with small botts..but one will still work in a 4wd after swapping the output shaft in one to a 4x4 version..
Anyone but me wish GM just used the same automatic trannies for 2wd AND 4x4's??--seems rather stupid to me,for them NOT to have just made the T-cases accept a 2wd style shaft after unbolting the tail housing...never understood why they made so many different ones,different length tail housings and outoput shafts...usually GM keeps things simple,this was one exception to that rule!~...