Also check the driveline hoop in Norm's picture, it can save your life! I like them back about a foot farther, but should be at least 2/3 forward.

Ha! Ha!
That driveline hoop weighed about 150 lbs, I lightened it up a bit and powdercoated it for FrankyB's rig.
FYI location: If it gets mounted back a foot farther on a W, it will hit when off road, due to suspension articulation....= )

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