+1 on fire extinguisher(s) on board, and make sure they're Halon.
If you're describing fuel smell and not exhaust, then I'd also surmise it's something leaking at/in the carb.
If it's exhaust smell, my KR has always done that. I've always attributed that to exhaust entering through the trunk somewhere. Seems to be more prevalent at low road speeds.
But, that just smells like the 1960s to me. Part of the old car experience.
I realize that you're a very knowledgeable Mustang guy, so let us know what you find out the problem is.