ARE YOU USEING THE THE STOCK THROUGH BOLT ON THE DRIVERS SIDE ?
If you want try this remove the drivers mount and buy a set of steel $14.00 mounts and install only the drivers side with a hardware store grade 5 though (long ) bolt in 5/8 inch size , this will cause the motor to be slightly higher on the drivers side and very strong also we have been running a 440 4speed b-body like this for years and also a 426 aluminum block hemi in a 71 challenger like this also no problems .