1965 Valiant LS Build

Personnally not a transvestite kinda guy, but I understand why people put SBC in most everything years ago and now the LS style/family of engines now.
One concern for me is the major surgery to the transmission crossmember. This is a structural member of the chassis. I would seriously consider constructing another farther rearward to aid in torque control/body stiffness, act as a driveshaft hoop and yet allow transmission removal.
Those 6 liter engines with the "bigger" cam produce a lot of torque, so body flex becomes an issue. Get into the local Mopar crowd and see what the factory big block cars had.
Also I would find an experienced welder to check your welding to verify the integrity. You do not want welds breaking when you get into the loud peddle. Someone in the local car community will have good welding experience. You may need to buy the pizza and beers.
Thanks for the advice, I'll definitely look into building some more support for the cross-member. As for the welds, my coworkers are very experienced and have checked over my welds. I definitely do not want them breaking/cracking under stress.