Everything I've heard, no they will not...
No it will not. Because it is a lock-up converter and the 350 turbo is not set up for a lock-up converter.
yes a 350 turbo converter will fit a 400 turbo and vice versa
If you're talking about a TH400 transmission, yes... the TH350 torque converter is one inch smaller in diameter than the TH400 torque converter (12" vs 13"), and you'll have to make sure the holes on the flexplate line up (or drill them for both patterns). You'll get a bit more stall out of the 350 converter. If you're talking about the 700R4, the answer is going to be no.... that uses a lockup converter, which will not interchange with the TH converter.
It might but keep a close eye on it.
not necessarily. depends on what it was made to do.
the transmission will bolt up to the block. trans mount in rear will most likely be different. shift linkage will sorta work. reason being is th350 has three forward gears and 700r4 has four forward gears. when i did the swap from th350 to 700r4 i made my own rear mount. instead of the stock shifting assembly i used a B&M megashifter for 700r4. make sure you use the torque converter for the 700r4. it should be noted that this is a common swap and many aftermarket companies offer shift linkage set ups or shifters for this application as well as mounts for the rear of the trans and any other adapting devices you might need.