Thanks for the link. No chance I would ever use JB weld as a fuel tank seal though. That is quite an unsafe idea, unfortunately.
I actually did the Monza conversion.
The set up is an external threaded sleeve and a cap that screws on the thread.
The sleeve has to opened up on the inside and fitted to each filler neck since they are all a little bit different.
I attached the sleeve to the cap with blue loctite then I put jb weld around the inside of the sleeve before putting it onto the filler neck.
Once it was installed and set up I then used water weld putty which is resistant to gas and oil and sealed the joint between the filler neck and the sleeve. After that had set up I used POR15 to further ensure a good seal and protect the water weld.
It may seem to be overkill but I have had no issues or leaks.
I looked long and hard for a screw in cap and was not able to find anything. It seemed that the only way to make it happen would be to customize your tank or have one made. $$$