It is common. But the bad smell is more to do with tuning than aerodynamics.
Modern fuels do not burn the same way as old fuel did. And it smells different. Some brands (like the current Shell V-Power) stink to high heaven.
Things get even worse if you have a longer than stock duration camshaft.
There are several things you can do to limit the smell in the car.
1) First of all customise the timing curve of your distributor so you can get much more initial advance.
The high initial timing advance will clean up the exhaust and stop it smelling rich.
2) Tune the carb so that you have around 14.5:1 air fuel ratio at idle.
3) On the motorways drive with the door glass up.
4) Make sure the PCV system is functioning correctly. Don't just use breathers in the valve covers and make sure the PCV valve is in the drivers side valve cover.
5) Make sure there are no openings in the bulkhead to let engine fumes in.
6) Make sure the rubber seal in front of the air vents in the cowl actually seal to the hood.
7) Extend the exhaust tip out as much as possible without it looking silly. if you dont have the chrome challenger dual tips then turn the end of the tailpipe(s) down.
Try fitting a T/A type spoiler to the trunk lid.
That is about all you can do to reduce the smell in the car as much as possible.