The fuel fill pipe enters the trunk area, and goes through a seal in the tank. There is also a rubber boot that seals the fill pipe to trunk floor junction. If you have holes in your floor, it's possible you are getting gas fumes from any leaky components there.
The '75 has a charcoal canister on the passenger side inner fender in the engine compartment where the vent runs from the fuel tank.
Check the hoses to the charcoal canister for condition, and look for any fuel leaks in the engine compartment.
Your carburetor has a line that goes to the charcoal canister as well. I'm thinking the vent line, purge line, or the canister itself.
That little charcoal canister get's pulled off by folks that think it's some evil Horsepower thief. All it does is keep gas stink away.
Other possibilities:
There is a rubber seal behind the sending unit that seals it to the tank. If you pull the sending unit and find the seal is damaged, you should be able to get a new one. Use vaseline or something similar to lubricate it for assembly.
If someone installed an early fill tube with a vent, that may be the source of the smell. Early fill tubes had a vent tube that vents the tank back to the gas cap area of the tube inside the trunk.
That gives you a little bit to work with.
CJ