The boxed centered crossmember should be correct for all firebird except th400 or non OE trans. should be same for camaro,nova,apollo,ventura,omega to 74
th400 crossmember was tubular with an extended pad for the trans mount. I think it was the same crossmember camaro used.

67 firebird used a different crossmember than 68-9, but i think it was just for the different linkage mount and otherwise fit the same.

Camaro BB 4spd had a boxed crossmember, but offset, or at least some did.

I have a th400 crossmember, modified for my 2004-r... /6cy