Ye I think the rubber part does bolt to the engine,and the fram on the car is solid,I will probably use aftermarket mountjimfulco said:6-cylinder frame brackets and mounts are totally different from the V-8 units made for that subframe. Big-blocks used frame brackets that located the engine slightly to the right compared to the small-block location. A BBC will bolt up to SBC frame brackets, but the exhaust and the steering box might be too close for comfort.
Transmission crossmembers are the same for 6 and small-block V-8, but big-block crossmembers offset the trans mount a similar distance to the right.
Just to be sure, check the bumper mounting area on both frames. A guy who had a '74 Nova told me there might be some difference in bolt holes or something, because the '74 Nova had the bumper mounted to shock absorber units, whereas the '73 and older had the bumper mounted solid.
It just occurred to me, the '74 Nova may have the clamshell motor mount system. What I said above applies to the older-type system where the rubber part of the mount system bolts directly to the engine.