Only use JB Weld if you can grind the rust right down to metal all around the crack. Also, check to make sure (if you haven't already) that the crack isn't in the breather tube...
I'd recommend at least trying the patch solution, because replacing the neck is a gigantic PITA. The rubber hoses feeding from the tank get really brittle with age and if you're not careful, they'll bend/break the metal neck and/or breather when you try to pry them off.
Or you could go ghetto and duct tape a funnel to a garden hose and feed it past the crack when you fill-up! I did that when I first bought my Rolla, until I could replace it to safety the car. Not fun...
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1990 5spd V6 Camry (Still kicking at 393,000km) 1991 Celica GTS -- Pappa needs a 3SGTE...and AWD for all this friggin' snow
Honda my A$$, you just can't kill a Yota...