[Axle]

I went the other way years ago in my '83 (700r4 to th350. And I'd do it again in a heartbeat).
Same yoke is used with the th350 & 700r4.
Your crossmember will need to be moved closer to the engine (approx 2 or 3", don't remember, and it really doesn't matter, bolt the trans to the engine and the crossmember to the trans; drill new holes).
If your th350 has a 9" tailshaft, you can reuse your driveshaft. If its a shorter tailshaft you will need a longer driveshaft.
You'll want to take a good stab at what speedo gears you'll need. Most 700r4's I've seen use the th400 style large diameter bullet driven gear, the speedo drive gear is interchangeable.
Check if there is an oil slinger (metal cup) on the output shaft (you'll know it if you have it when you pull the tailshaft to count the speedo drive gear teeth if its already got a gear installed. If it doesn't have a slinger, clean the vent hole in the yoke and plug it with silicone otherwise you'll leak trans fluid out the yoke.
The rest will be 700r4 related bits 'n pieces. And you have already checked into the geometry of putting the tv cable attachment point in the correct location on the throttle arm? Its not the same as the kickdown location on a th350, you can safely put a th350 kickdown in place of the 700r4 tv cable attachment point, not so much the other way. Never, ever drive the truck if the tv cable is disconnected. Never. Or you'll be removing the trans shortly afterward.