New Fuel Tank / Seal yet massive Leak after filling my Tank

And maybe, fill tube not installed at correct angle ??
I`d go w/ this ^^^^. Try loosening the tank straps and wiggle the tank around to see if u can feel for the best fitting position for it. U did lubricate the filltube and the rubber seal when installing it didn`t you? Of course u cold have bad rubber , but I doubt it, if u bought one specifically for ur car.!