There are definitely different input shafts. You need to read this article from Advance Adapters.
http://www.advanceadapters.com/tech-...w-process-205/
“In 1971, New Process introduced a model NP203 (chain-driven), and NP205 (gear-driven) transfer case. Both of these transfer cases used various input splines. As a general rule, all of the transfer cases that couple to a TH350 automatic are normally a 27 spline input; when coupled to a manual transmission a 10 spline input; and when coupled to a TH400 a 32 spline input.”
If you read the next few paragraphs you’ll also see that there were different bolt patterns as well, figure 8 from 1971-1984?, and circular bolt pattern from 1985-1991.
And then there are also 2 different stick out lengths for the input shaft.
Something else in this article makes me question if the 4wd 3-speed trucks used the NP205, and makes me wonder if the 3-speeds were using a different t-case, maybe even a divorced unit?
“The 1971-79 NP205 was only found mated to the TH350 and the SM465 transmission”
That being said, I feel like more 4wd came from the factory with a 4-speed manual than did a 3-speed manual.
Best of luck and keep us updated!!