salvage yards have the slip yoke.
tci
moser
martel brothers

any driveshaft specialist or transmission specialist should be able to get one. if you take your driveshaft to be shortened, they should be able to get you a yoke. driveshaft specialists can sometimes be found in the yellow pages.

or someone here might have an extra th400 driveshaft.

if you were swapping to a th350, or 200-4r, you wouldn't have any driveshaft issues

IMO dont go any lower than 1600 on the converter. if the 2spd converter has a 3 or 4 digit GM code on it, and/or a part #, i might be able to tell you it's stall speed. i think the 2spds might have had a decent stall to get the low torque first gear moving, so hopefully you can reuse it.

unless you have a really hot cam in there, you probably wont want to go over 2000-2200 stall speed.