i am no truck expert.
with that said, i "think" the transfer case was a divorced unit up until around 1976 or so. these also had the shorty transmission tail shaft that used a bolt on yolk for the driveshaft to the transfer case.
so looking at that, what year is your frame ?
if the early transmission and transfer case is available, can it be adapted to your frame by possibly using the early cross members ?
i can't answer about the newer cases being part time units. others will have to answer that question.