That is what I am going to do when my current 700R4 goes. It is pretty easy if you plan on sticking with the 208 tcase. The Th400/208 & 700R4/208 combo are the same length so your driveshafts will work with no mods. The difference is the 208 is a 32 spline behind a Th400 & it is a 27 spline behind a 700R4. You can either get a 32 spline case or swap the input gear of the 208 to 32 spline to get it to bolt up to the Th400.
Beyond that you will just need the standard parts. Tranny, dipstick, dustshield, possibly crossmember, & figure out how to shift it & get the kickdown to work.
Harley