[Dads 72 Malibu]

First thing I thought was the timing may need tweaking.
But the aforementioned questions are also worth looking at, as are the plug wire location/routing and condition; spark plugs themselves; distributor, cap & rotor; carb tuning/size; bad/dead cylinder.

I fail to believe that a new set of motor mounts would CAUSE an engine to vibrate.
They may exacerbate an engine problem.
But be THE cause?