Not having a crossmember was never a problem for us when we rodded 55-56 Chevy's in the sixties. My 56 had a stout 292 (283 over .060) with a Muncie 4 speed. I used to power shift the snot out of it and it never gave me any problems.
It is a simple matter to fabricate a trans mount out of tubular steel if you are so inclined. Please do not use angle iron however, it looks really tacky. If you decide to add a trans crossmember you may as well go ahead and convert to side mounts on the engine also.