If a rear crossmember and mount is added to a to a car or truck that came with a transmission cantilevered off of an engine with a front mount and two rear side mounts, one now has what is called an "indeterminate structure", and it is quite possible that the loads on the transmission to bellhousing ears and bolts will be greater than without the added rear crossmember. (this may be the longest sentence that i've ever written - sorry).
If one is worried about the load on the upper ears due to the weight of the transmission, a quick and dirty calculation will show that that load is less than the load on the lower ears with a forward engine mount and an aft transmission mount (like a pre-1952 Chevy passenger car). I have read that the "best mounting arrangement is center side engine mounts and a rear transmission mounts. This results in low bending moment and loads on the transmission to engine attachment joints.
The 1955-57 Chevrolet passenger cars came with the cantilevered transmissions. Has any one seen the ears break?
Have we ever determined just how the 1965 truck in question was originally built? if it came with the cantilevered transmission, there should be nothing to cause concern. If it came with a rear crossmenber mount, then there may be some changes to be made.
Hoyt
