Don't know about the mounts but my 85 5 speed put up with my DOHC just fine but then again I did not do a lot (ok NO) clutch dropping starts. The only thing I found on mine was that the spread from 1st to 2nd was pretty big and the shift was 'slow'. I'd be screaming in 1st but by the time I got into 2nd the revs had all dropped off. Your mileage may vary in that respect, that was just my experience. It was nice on the freeway though, nice low rpm at 70 mph but oodles of power to just pull away when needed. I'm sure the Sc would be similar.
Personally, I think that if you've got a good reliable Fiero of any sort, it'd be a shame to spoil that.
Anything I might say is probably worth what you paid for it, so treat it accordingly!