To use a 700r4 in a 67-72 4x4 where a th350 currently exists you need the following part:
http://www.advanceadapters.com/products/pn-50-6900/
Like others have said take your core 700r4 to a reputible shop and have them rebuild it with the $165 output shaft installed from Advance Adapters.
Then when you get your rebuilt tranny back remove the engine, remove the th350 out the front, and leave the NP205 and adapter bolted in place. The 700r4 is about 2" longer than the th350 so you'll need to move your engine crossmember ahead that much. Remember in a stock 4x4 the engine sits farther back. I HAVE NOT DONE THIS MOD but others on this board have. The other option is to move the t-case back but then you need to shorten the rear driveshaft and lengthen the front one.
WHEN I do this mod I'm moving my engine forward.
Check for that AA part at Summit Racing and other suppliers. The last time I checked it was cheaper there than from the manufacturer.
As far as I know there is no direct bolt in overdrive replacement for a stock 4x4.
Also when doing this mod seriously consider a new drive sleeve if yours is grooved and new oil seals in the adapter. This is heavy stuff and you don't want to go in there twice.