I would suspect the front end damage or misalignment in the frame as the problem. There is no left/right mount. I don't know if there is a difference in the mount to frame supports.

You have multiple issues that come onto play: The welding of the frame--why weld in chebby mounts when cramaro mounts interchange?--the obscure mention of chassis repair on the car and the obscure mention of collision damage on eigher your car or the donor car. I would foucs on these issues more so than the mounts themselves.