My '37 is the same way, nothing stands out as a problem, its just not as clean as my late model Toyota or VW. On mine, I would say its the way the fuel smells coming out of the intake just as you shut it off. The inline six has long intake runners and the fuel has to go some where, so it vents out through the carbs and air cleaners. Nothing you can do will stop it, its not a leak that can be fixed. Once the engine cools down, the smell goes away.
One thing I did to stop the fuel tank from venting vapors was add a charcoal can to catch the odor. It works really good and cut down on a bunch of the smell. I got hold of a charcoal vapor can from a older car at the salvage yard that held a bunch of the pellets. I dumped it into a pickle jar with two tubes soldered in the lid. Air in is a short tube, air out is long enough to reach the bottom. On the end of the long tube is a small fuel filter to keep from sucking up carbon pellets. As you drive, fresh air is pulled over the pellets to clean out the fuel vapor from them, when it sits, the fuel tank vents out into the carbon which absorbs the odor.
Joe
