'79 K20 trans help

[1FaastC10]

well, it turns out that the trans in my old '79 K20 went bad. and considering the guy i sold it to has been one of my best friends for over 15 years, im going to help him fix it. we got it running 2 weeks ago, and backed it off of the trailer. the trans slipped a little at first, but we just assumed that it was due to the truck sitting for almost a year and a half. it drove and shifted fine. he decided to check the fluid, and it wasnt registering on teh dipstick. 5 quarts later, it was at the full mark. he pulled the pan, and found lots of shavings, and now it has no reverse. so we're looking for a trans for the truck, and need to know exactly what we need to be looking for. truck is a '79 K20 with a TH400 and an NP203. will any 4wd TH400 work? or does it have to be one that was originally mated to a NP203? anything else we should be watching for? thanks for your help.

[1FaastC10]

ttt