TH400 in place of a 700r4

[thirsty]

What does it take to swap a 700r4 out with a 400?

I found a 85 k10 350/700r4/208 with a real decent body I want to make an offer on.  Pretty sure the tranny is junk. I could probably get a 400/208 combo reasonable out of an 86 (I think that's what it is). Is there any difference in the length of the transmissions or is that made up with the trans/transfer adapters. The 400 is in a lwb and the 85 is swb. Could I use the same drive shafts? And the kick down is different also isn't it?

I'm not much of an auto tranny guy  so thanks for any input.

[Captkaos]

Kickdown is electric on the 400, the mount on the 700 vs the 400 should be the same or close.