When I did my truck a crossmember needed to be removed and replaced with a transfer case one about a foot farther back. I had a complete 65 donor truck and installed the 4x4 stuff on a 68 3/4 ton long box truck.

There is a cross member on my '61D200, '68 D100
and '65 D100 that "drops" under the drive shaft.
It is, in the general location, where you'd put
your t-case cross member. If your frame has this
member.... it'll likely have to be removed. There
are 2 brackets & 8 bolts per side needed to hang
that NP201 cross member.
John