Went thru this a while back, what a pain in the ars. There were no new tanks to be found anywhere. Thinking if I got one from a yard, how much longer til those tank fittings crack, also my tank is in good shape, other than the one fitting, why waste that. Considered JB 2-part marine grade adhesive but decided I wanted this to be a 1-and-done situation, so got off my wallet and bought AMS-S-8802 B2 fuel tank sealant from aircraft spruce for $37.50, (1) 3.5 ounce syringe will do all the fittings. B2 is a nasty-awesome fuel tank sealant that sticks to everything, cures to a hard rubber and lasts for decades.
Here's how I went about it (now 1.5 yrs, no issues)
1. Cap off water tight all openings and scrub the crap out of the top of the tank around all the fittings with Dawn soap, get rid of all oil/grease/fuel residue. Rinse and Let dry fully.
2. Lightly scuff all around each fitting with sandpaper or scotch brite. Do all the fittings while you have the tank dropped, there might be another crack waiting to open up.
3. Rubber gloves... Mask off connector nipples. Mix the B2 sealant per instructions, and spatula around fitting base flanges with a little extra pressed into any cracks.
Working time is 2 hours, cures to usable condition in 24hrs.
4. Ensure all fuel line connectors lock properly; trim sealant if required.
5. Install tank and get-to-gettin!
Cheers!