I have/had the same problem with my 67 coupe.
Found the fuel tank was fitting like a dick in a hat.
Sealed the gaps and that improved things a lot. Not fixed totally yet,I just keep filling every seam and hole I can see.
Put a light in the boot (trunk) and seal where the light comes out do it at night or at least with the door closed (garage not car)
If at first you don't succeed , find a bigger hammer
Found the fuel tank was fitting like a dick in a hat.
Sealed the gaps and that improved things a lot. Not fixed totally yet,I just keep filling every seam and hole I can see.
Put a light in the boot (trunk) and seal where the light comes out do it at night or at least with the door closed (garage not car)
If at first you don't succeed , find a bigger hammer