I had a similar exhaust issue on my 58 Suburban when I bought it. The rear clam shell seals were shot as were the sliding glass seals.
The exhaust was pretty much roached so I took to a local guy in Missouri and had him custom bend and weld up a new system for me. When I mentioned my exhaust smell problem he said the same thing about the fumes getting vacuumed into the cab through leaky seals. He recommended I run the exhaust tip out the drivers side of the truck. It now exits behind the drivers side rear wheel and sticks out about 4 inches. It solved my problem. I have since replaced the weather stripping in the back. The guy did a real nice job and was even careful not to interfere with the spare tire hanger/storage area under the floor.
