My tank has a sealed cap, it's in the bed floor so it fits flush with the wood. All venting of the tank is through the charcoal, out when it heats up, in when using fuel, this keeps the charcoal purged.
Another place to check for fuel vapor is the intake manifold. Inline sixes tend to pool fuel at the ends of the manifolds where there is less heat. If the carburetor is running rich due to poor adjustments or wrong metering, this makes for a smelly engine as all the extra fuel evaporates out of the intake runners. A nonworking heat riser doesn't help either.
Once I installed a Air/fuel ratio gauge, I quickly figured out mine was running pretty rich. I leaned it out with a metering rod change and mixture adjustment, most if not all the smell went away. I used to have to let the truck set outside after any driving so it would air out, now its straight into the garage except on the hottest days.
