Dumping exhaust down under the car

3" pipes to the rear with tubs, and inboard springs is quite common.
there are several posts with pix here.
You need to find a craftsman that WANTS to do it.
Cheers
I forgot to mention the one year only 67 tank and that we are using mandrel bent pipe. No one around here even has a mandrel bender. We mocked up the front half of my exhaust using drier ducting and expanding foam, and then shipped those pieces out of state to be copied in stainless. You were right about one thing, you need a craftsman.