Thanks for the pic Scott. I wanted to see what your crossmember looked like but had never asked or gone looking.
One approach I've considered is tying the boxing into the crossmembers it runs between. I've not thought through stress transitions, though at the front I don't think it would turn out to be a problem.
As I drove home from work today I was thinking through the boxing process and what it might look like, and realized there might be another approach, borrowing from Grigg's suggestion above.
I have a complete 1-ton frame in pieces, which I was considering cutting up to form the box plates. But if instead of cut lengths of that frame to match up to the original frame I can form a box using bolts instead of welds. This is attractive to me in that it makes it easier to remove should I ever make that choice.
I'm picturing a couple of bolted in box sections mounting the front engine mounts for the Cadillac. The box section might only be 18 inches long, but would add strength front the front crossmember rearward behind the engine mount. This would be the area most stressed by the added motor mounts.
The rear boxing might be a little more complicated but I picture something similar, perhaps in two pieces (on each side). One piece would add strength around the spring perch where I'm removing a crossmember and the second would be in the area where I'm adding a crossmember and back far enough to support the rear of a crossbrace.
I have a pretty clear picture in my head, but nothing on this computer with which to draw that picture.
