Look up Caswell gas tank sealer. Special type of 2 part epoxy that is gas/e85 safe. Designed as a fuel tank sealer/liner but I used it in a small batch to seal a "custom" (read homemade) electrical bulkhead for my pump wires. Cost around 50 bucks for the kit.

 

I ended up just ordering a stainless steel bulkhead fitting used for hydraulics. Kinda pricey but at least I can have it welded up to the steel hat instead of using JB Weld.

 

I find silicon brazing stainless fittings to the hat easiest. It can be welded no doubt, but tig brazing it or even hi temp metal solder is stupid easy without worrying about burning thru or warping the hat..