The Cadillac engine mounts are pretty far forward. A bit more so than a Chevy V-8. I'm not aware of any commercial mount kits designed for this chassis and expect to be fabricating my own. The front mounts will be close to the front crossmember and I think close enough that it will be well supported at front.
The Caddy mounts with the two front side mounts and then just the transmission mount. That means the crossmember I'll be fabricating for that mount will be important and will need to be mounted solidly. I don't believe an open driveline should transfer any push to the transmission but it will twist pretty hard, so I'm thinking I'll build an open x-member from the (front) rear spring mount to the k-member and use that to support the transmission crossmember.
That coupled with boxing the frame should stiffen things pretty well. I'm wondering if maybe I don't need quite as much box and could maybe just box where I need to add mounts inside the frame. But then I'm more than doubling the horsepower and torque with that Cadillac and I do intend to pull a heavy trailer with this thing.
I do not intend to box the frame back of the k-member. I see no reason to do so, and it simplifies plumbing, wiring and mounting the hitch.
The open x-member should allow room for the exhaust to pass through.
That's my thinking as I've read responses and now begin to fade tonight. Please offer any critique and I'll keep expanding my description or changing my plan until I no longer hear any issues that concern me...
Thanks!
