Just installed new fuel tank and believe the best solution to a leak free system is 2 things.
1. IMO new tanks are not compatible with old filler pipes and best solution is to replace the filler pipe also. I tried the oversize 0 ring and it was better but some seepage. The new filler pipe was a snug fit and leak free.
2. As previous owners have stated to get a leak free fuel sender you need the cork gasket, copper washers and aviation sealer.
I've put new tanks in about a dozen trifive's in in the last few years and always used the original filler neck, never a problem.
1956 210 delrey sedan 383 700r4 12 bolt posi
1968 Camaro 468 bbc power glide 12 bolt
1968 Camaro 408LS power glide 12 bolt super gas
1971 el Camino 383LS 700r4 12 bolt posi
still searching for the right 56 nomad project.