A solid trans mount is asking for trouble. I have a very stiff tube chassis, yet use shock biscuits, four of them, to allow give for chassis flex. Build it to put some upward pressure on the trans, (about enough to support its weight) and let those shock biscuits limit the loads the case has to put up with due to chassis flex. And i don't care how good a person thinks their chassis is, Everything bends some!
Greg i am using a polygraphite mount from Mancini Racing and fabricating my own cross member, the 1/2" front motor plate will also be rubber mounted too. Yes big blocks twist everything my 440 proves this !! I still love my .528" mopar cam