Is the stock type transmission mount strong?

Installing my 408 and 727 w/transbrake in my '68 Valliant. The transmission mount looks to be held in the crossmember with just a small 5/16" stud inside the mount to the crossmember, held with a nut. This just does not seem to be a very stout setup. The poly mount does not even fit snug into the crossmember. This is a steet/strip car leaving on a transbrake. I'm more used to the GM type held in with large bolts. Am I missing something here?