Thanks for the replies guys, getting more brains involved is what I love about bringing ideas here.
I agree boxing the frame could add some complications and holes in the plates would help. I also think welding some nuts inside the frame would be part of the answer.
After looking the frame and visualizing it boxed I see the boxing as starting with the front crossmember and extending back to the third crossmember (the one under the back of the cab). That is about seven feet of boxing adding strength and stiffness through the area where the motor sets.
I'm somewhat relunctant to hang something real low below the frame between the axles. The transmission crossmember to support the Cadillac Turbo 400 will hang down a bit, but not as far as anything that could clear the torque converter.
The stock bellhousing crossmember is completely behind the center of the rear spring perch but only by an inch or two. A crude estimate is that the correct location of the crossmember to support the TH400 will be about 30 inches behind that same location.
I picture the new transmission crossmember fully below the frame, probably bolted to the bottom of the frame rail. Then a couple of diagonal supports going forward to the area of spring perches.
I'm liking this idea, but would rather hear other views and suggestions before I commit myself...
Thanks!
