It did not take long before the dog bone motor mounts failed in my GTX. With the amount of mods performed, including a firm shift kit for the trans, the engine began rocking on its mounts. This caused the headers and exhaust to make some noises. I replaced the dog bone mounts with a solid plastic material type. You can remove the internal rubber and replace it with the solid neoprene like material mounts. I feel some vibration from the steering wheel at idle, but I had the same feel with my 68 Firebird and turnbuckles to hold the engine down from the big torgue of the 421S.D.
The performance sites should carry the neoprene mounts.