I've never experienced that with any of our builds here. Yes, the key is planning and sealing. If you're sloppy in sealing things up, then you will have issues. Every little gap needs to be sealed.....along the roof line, the little alum close-out panels that span between the chassis and body next to the fuel tanks needs to be 100% sealed to the body and the chassis, cockpit sidewall panels behind the door openings, under the stereo, around the AC vent tubes, the footwell panels under the dash, etc. Keep in mind that the air from the engine bay (with the exhaust fumes) get drawn forward into the tunnel and exit out the front of the tunnel, so everything in the tunnel needs to be sealed. All of that needs to be done during the build. Most of those places are not accessible on a finished car, so you need to get all of that stuff sealed up as you're building....not after it's all done.

This also points out the need for getting the most airflow thru your engine bay as possible. If the exhaust fumes are that strong, it tells me that you have very little air flowing thru your engine bay.....getting the hot air out and fresh air in.....all of the heat and exhaust fumes just swirl around back there. As I mentioned above, the tunnel pulls air from the engine bay, so it sounds to me like you have lots of negative pressure in your engine bay, which sucks the exhaust back into the engine bay and then forward up thru the tunnel. With proper venting of the engine bay (lots of paths for fresh air to get in and lots of paths for hot air to get out) there is no way for the exhaust fumes to build up in the first place as they will be "blown" out of the back of the car instead of being sucked in.