" . . . i'm missing the lower dust cover for the bell housing/flywheel. is it ok to install the motor and put that on after i locate one? if not and it needs to be in place, "
Yes, it is OK to install without - and/or put one on later (GM thought it was needed).
" . . . how do you get the tranny bolts through the bell housing?"
Install two upper transmission bolts from the outside; and, two lower transmission bolts from inside the bottom of the clutch housing.
" . . . should there be a rear crossmember supporting the tranny?"
Yes - there should be a rear-transmission crossmember on 47-53 1/2 ton trucks.
Many were removed when a transmission was removed (and, the "mechanic" failed to put it back on). Some people (including GM) think it is needed.
There is a rear-transmission support/mount assembly that mounted on that crossmember. Several suppliers sell that mount/assembly.
