Hi Chuck,
There's several sources of cabin smells. The firewall panel on early cars is one of them and easily fixed with a filler panel.
Also the side vent windows seem to allow exhaust smells in at low speeds < 45mph so I keep them closed except when parked in the sun.
Weather stripping can be improved by adding more to close gaps especially around the cowl to front of the doors, no exhaust smell there but lots of heat/cool loss.
The single biggest improvement you can make is to provide outside (fresh) make-up air to the Heater/AC unit. There is none as stock meaning as you drive the air bleeding out of the cabin creates a low pressure zone inside and fumes will find a way in.
Directly pumping outside air into the Heater/AC stops this by raising the cockpit air pressure. Also without make-up air the cockpit gets stuffy and humid, not good for long drives. With the A/C on your getting cold, dry stuffy air...
There's no easy way to feed the make-up air, however what we've done on all our cars is plumb the right side NACA Duct on the hood to the Heater/AC through the side of the cowl. Very tricky and time consuming but worth the effort.
There are no doubt other ways to do it, the key is to pump clean air directly into the blower, just getting some into the cockpit is not good enough to ensure fresh make-up air being mixed with the air already in the car.
As the factory CAV unit has four openings on the dual squirrel cage fans and we've dedicated one to make-up air, the ratio of recirculated to make-up air is about 75/25. This is greater than production cars however we need more fresh air since there's no cats and most of the CAVs have bladder fuel cells which are a bit smelly on their own.
Take a look at the pics, you may be able to copy this, just leave yourself lots of time
At the end of the day, it will be worth it.