1968 Chevy Chevelle exhaust smell in interior

Hi:
I have a 1968 Chevelle Malibu which I restored (frame on). I installed new floor pans, trunk pans, inner rear wheel tubs, quarters, trunk floor extender corners, as well as a bunch of braces. Seam sealer was used on all mig welded joins. New noise reducer insulation on the floor, new carpet. New exhaust from front to back. New manifold gaskets. I am runing a 403 olds with TH350 trans. I get an exhaust smell in the car while driving around 30 to 40 mph with the drivers window about half down. There are no hoses leaking on the engine, no exhaust leaks and the tail pipes extend 3 inches or so past the rear bumper. I have checked for entry points all around the trunk, quarters, wheel tub area, and there are no holes anywhere. The only possibilitie is the trunk cover weather stripping which I will be replacing this spring. I know that there is a vacuum created behind a car as it travells. Somehow the exhaust is getting sucked into my car. Can you suggest something I may have missed. Thanks !