I don't know if this is what you need to know or not, but I have an easy way to tell if you have an NP205. On the back of the T-case look in the center of it. The 205's have a small circular piece of metal with 3 bolts holding it down. Its almost like a big silver dollar with 3 bolts in it. Anyways, the 205 is the ONLY one with that. Its how I tell the difference. I don't think I've ever seen a Dana 20. They mainly went in 3 speed trucks with smaller engines. And the 203's are easy to pick out just because they're so big.
'73 K5
Chevy good...Ford bad