'73-'76 Duster Big Block 727 Tranny Crossmember

My guess, without pictures, would be motor mounts installed on incorrect side of the flanges on the block or transmission mount installed wrong direction.

You don't need a hosting site for pictures. Compress your pictures and post them. I believe there is a thread on here somewhere that tells how in the website help section.

Good luck to you. I hope you get it figured out.