What is the best way to seal the cross over between the petcock and tank on the El's is there something better han te brass washer?

Copper crush washers !

Yes copper gaskets one on each side of crossover.Watch aftermarket crossovers esp if chromed.Removing the chrome may be required to seal-all mating surfaces.Also paint between tank and copper seal may also allow a leak.These crossovers must lay unloaded as flat as possible.Seen some bikes where more than one seal is needed.Buy some spares.