I mount all my engines by using a rectangular piece of lead between the mount part of the case, and the frame tube. Also, a piece goes between the "U" clamp part and the tubing.
I bed, or "glue" the lead in with PC-7 (like JB Weld, but thicker) to even out any discrepancies in frame angle/mount angle so the engine is fully mounted on the frame.
I do this because I have noticed that anything but a "perfect angle" results in a high stress area on the frame. A lot of cruiser type frames leave a wedge shaped gap in there, not readily noticeable.
I put the whole works together, let the PC-7 partially set up, then tighten the mounts down until the PC starts to squeeze out. After it is fully cured, I finish tightening the mounts.