There are versions of the TH350 that can drop right in place of the 700R4. In fact, I just yesterday removed that output shaft and spacer (goes on the tail housing before the t-case adapter), and made mine back into a stock 4wd TH350.
You wouldn't need any driveshaft mods then. The trans mount holes might be off by about 2 inch or so.
If you wanted to find those parts, I have the spacer block, Tranny tom has the old shaft.