Other then an obvious leak, which any one off, hand built car is susceptible to, the most suspect place is the gas tank vent. Somewhere here I have a fairly long thread from a couple years ago that I went through several tank vent "designs" to eliminate those fumes. I tried to find the thread, but it's too early After spending hours working on the problem, I still have a little gas smell if the car is brought in after a run, but it does go away over night. The car is in my basement shop this winter and with an almost full gas tank - not a sniff of gas (and guys, no preaching on those dangers, it was supposed to have been drained, but CRS set in and I put the winter "door" in place before it happened)