I have 9300 miles on my restored 69 road runner & I always have a slight smell of exhaust fumes in the passenger compartment. I tighten the header bolts and it usually decreases, but this winter I've been driving more & since it's cold the windows are rolled up & the exhaust smell gets so bad my eyes burn.

How can I locate the leak or leaks?

When I had this happen, it was from exhaust entering through the trunk seal. I added cardboard panels over the wheel wells to prevent air movement and it helped tremendously.

Check to be sure there is a partition between the trunk and the backseat as well.

I installed a "bolt on, pre bent" system and couldn't stand the fumes.
My exhaust shop pointed out that the tailpipes ended just before the bumper. Fumes were entering through the trunk. He extended the pipes just past the bumper, no more fumes.

My exhaust fumes (and rain) were entering through the tail light area.

So I put a set of halogen lights inside my trunk (power cord thru side marker light opening) and shut the trunk lid. In a dark garage, the light showed through all around the tail light assemblies!
So I removed the stock gaskets and went to my local hardware store and bought a couple of rolls of the 3/16" or 1/4" adhesive foam insulation. I recreated the tail light gaskets and the fumes (and rain) are gone.

Another area you may want to check is where the trans shift linkage comes up through the floor of the car. There should be a rubber boot there to help seal the hole in the floor.

Excellent ideas, I was focused on the firewall alone, I hadn't even thought about the back end of the car.

I will work on those areas moving forward.

Thank you all very much for your feed back, greatly appreciated.