[mrein3]

The piece you are talking about is called a transmission to transfercase adapter. As recently as 5 or so years ago I was able to purchase the adapter for my truck right over the counter at my local Chevy dealer. However my adapter was for the NP205 to th350. All the adapters are different and are specific to the combo.

My father-in-law has an 85 or so 3/4 ton pickup. I'm not sure what specific 4-speed transmission he has but it for sure has a NP205 so you MIGHT be in luck buying new.

I don't admire the job you have in front of you. Those are some heavy pieces you are talking about.

On my adapter there are oil seals on each end that the coupler rides around on. They keep the tranny fluid in the th350 and the gear oil in the transfer case. Because I didn't want to go back I got a new coupler too. I'm not sure on the 4-speeds but you may want to investigate and consider that too.

One more thing as long as I'm data dumping on 1972 transfer case adapters...
GM made all transmission/transfercase combos in 1972 the same length by having different lengths of adapters. Long story short, if you want to ditch the 4-speed for a th350, you can make the swap without any other changes. The driveshafts on all 1972s are the same length and all use the same crossmember under there. HOWEVER to do the swap you need to get a complete combo. For some reason the manual transmissions had 10-spline output shafts and therefore the NP205 has 10 splines on its input shaft. The th350 has 28 splines on its output shaft and needs to be coupled to an input shaft with 28 splines.