Gas fumes in cab

[ssuesens]

Recently acquired a '71 C10 Custom with the gas tank behind the seat. There is a relatively strong gasoline odor in the cab which I obviously want to get fixed IMMEDIATELY! Upon inspection I found old, tired out silicone caulking between the filler neck and the tank. I believe this is the source of the vapor leak. Aside from removing tank and filler neck to have it professionally soldered or braised, it there a product (better than silicone) which I could use to seal the area? I'm thinking something similar to JBWeld or MarineTex but must not be gas permeable.

Thanks!!!