Hi And welcome Rrichar1
I've used (sucessfully) JB Weld on a cracked engine block and a ton of other things, but gas tanks are nothing to fool with. It's difficult to call the fire dept when you're 20 miles out to sea. Look and read the instructions on the JB Weld (or better yet, call their customer hotline) and ask them if it's effected by gasoline after it cures. Personally I would not use it, but that's just me. They make a liquid liner that you can use in gas tanks to seal them, but it requires you to slosh that liquid all around... impossible to do if the tank is mounted in the boat. And if you're going to remove the tank to use the liquid then you might as well just replace the tank. Don't buy a fiberglass one, for reasons expressed in other posts here. Alum is ok, but make sure you use a filter before the gas goes into the motor. Stainless is the way to go, but it's expensive.
I use a Hydrofoil on my Johnson 200 and I agree with you that it makes a big difference.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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