When mounting the rear of it it should be in rubber or on a leaf spring such that the mount shares the weight but doesn't restrict the movement much. The engine is mounted in front and side mounts on the bellhousing, those are your primary mounts. If the back of the trans is mounted too solidly you'll break stuff as the frame flexes and or have vibration issues.
Curious what the factory mounting solution is if you can find a parts diagrams or pictures of an original one?
Also, I agree that a u-joint is not the ideal coupler because in this case it wouldn't have any angle to operate through, it'd be straight. A U-joint with no movement soon wears flat spots on the needles and corresponding groves on the trunnions. I suppose if it's a bushing type u-joint (no needle bearings) there is more leeway, still though that's not as good a junction in this case as a straight splined coupling would be.
Grigg
