You said that you lost one of the gas tank mounts, do you mean one of the studs?
If so, the extra rattling around while it was falling out may have caused the welded in nut to get a hairline crack around the welded area and slowly leak. These tanks are known to leak from the welded in nuts, brand new out of the box. It does not happen often, but it is not unheard of. They can be that weak.
I do not know the size of the studs, but I'll look at my tank and compare a stud on it to one of my metric taps.
Edit: I re-read your post and it sounds to me like you tried to use electrical tape as a thread sealing tape on the fuel valve threads. If so, it will leak, guaranteed.