you will need to get a different tail shaft, the th350 is shorter, and to be honest, you want to the 700r4, get it rebuilt if you have to, the difference in fuel economy will pay for the cost of the transmission in time

350 hydromatics come in 3 tailshaft lenths (sic) 6 inch 9 and 12 they have the same output spline flexplate bolt pattern and engine mounting pattern to swap out a 700 r4 you need a 9 inch you will have to move the transmission support forward several inches .this causes a little alinement issue with the shift shaft in some applicatinos but it will work other than having either not enough positions or too many on the shift quadrant , and you better hope that you are swapping out an early 700 with a speedo cable as some later units(i think) had a speed sensor that means finding a new tailshaft cone and gears to drive the speedo cable there is a cable operated shift control on both transmissions and a vacuume operated pressure control on the 350 that warrants some understanding .also late 350s had a locking torque converter(adds an electrical ciruit) in an effort to improve fuel economy (but they also cut the weight out of some of the internal rotating elements at the same to reduce weight)some would say and reliability i'd call it an easy swap for any back yard mechanic certainly much easier than what you need to know to repair cars of only one decade later