So... finally, I got under the thing and noticed fresh gas had trickled down from the top of the tank and was collecting underneath. Not enough to drip and wet the pavement, but enough to detect by odor.
Now, a little background. About four years ago I pulled the gas tank to inspect it, empty out any debris, and renew the hoses etc. Which I did. At that point I noticed the big round fitting on the top, than holds the gas gauge sensor and fuel line (I think) was kinda loose. But I couldn't see any way to tighten it and I made a mental note to follow it in case it started leaking. Well, sure enough, it's definitely leaking now. It seems to happen more with a full tank, which indicates to me it's the top fitting that is the problem.
Not having pulled the top fitting, I don't rightly now if it's a defective gasket or some other problem, like just being loose. Since it's an older vehicle I'm thinking the gasket is probably shot. I am prepared to cut a new gasket, but the question is where to get the material for that? I figure neoprene (not nitrile) rubber is the minimum required, as it needs something to withstand modern gasoline. In the past when MTBE was being added to California gas, I understand a lot of old rubber parts in fuel systems rotted away. I have a '78 pickup that had that disease, had to replace every bit of rubber in the fuel system. But I never did see that level of rubber rot on this van, although as I said I replaced all the tubing I could without pulling the fitting on top of the tank.
Any suggestions? Any places I can get some neoprene rubber? I figure I just need about a square foot, maybe 1/8" thick. I'll know more when I jack up the van and drop the tank again.... either this afternoon or tomorrow morning...
I do have some odd pieces of rubber knocking around here, including a piece of red ... which might work. Might have to soak some samples in some gasoline overnight to be sure...
Will also call a few auto parts places.... but most of them only know enough to ask "What year, make, and model?" which of course is a non-starter usually for this van. Most parts houses don't even list the '67 as having a V8 motor...