Dont think you will be able to see anything without dropping the tank completely. I've pulled the tank off my 240z. Could send you a photo so you know what you are dealing with before you attempt it if you like. If you remove the rear internal trim pannel you'll be able to see the vent pipe. It goes through a grommet down to the fuel tank. Its probably perished where it goes on to the tank, if its leaking where you say it is. I had a similar problem with a commodore. It had little hoses all over the top of the tank to let pressure equalise i guess, but they had all split, so if i filled the tank too much it would just leak out. And the petrol guage didnt work, so it was a real pain. I pulled the tank, replaced the hoses, and removed the sender guage, unhooked the anti petrol theft coil that had fallen into the tank and wrapped itself areoung the float. Put it all back together, and no more leaks and the petrol guage was working again.