I had many issues installing in a 68 340 barracuda.But I could see that it had been in an accident sometime in life.I shimmed the driver mount like you did,But I aslo slotted the frame and moved the engine back a little bit 1/4 inch maybe.been ok.loosen the trans mount bolt also,slide back.I also installed the shumacker engine bracket for extra caution.
Make sure the mounts are on the correct side of the block,also the drag link isn't flipped around.