When I made my engine test stand, I used a part of a cheap Harbor Freight engine stand (the part with the length of 3" tube, with a plate welded on with the 4 mounting fingers) as the rear mount. It is adjustable (I have had a flathead, an Olds Rocket, and an early hemi on the stand so far) and has worked great for me. When I mounted the hemi, I had to move the mount back a bit, but I just drilled another set of mounting holes in the base.
Something to think about.