There are some very good silicone seam sealers available today that will hold up well over time. One thing to do before you start sealing seams is to make sure that all new and old metal surfaces are properly sealed from rusting with primer and paint. If you put a seam sealer on bare metal, you will have the same type of rust problems in just a few years, although it will take many years before they become as bad as the ones that you have already repaired. Keep in mind that the old metal needs to be protected as well as all the new metal. Before you finish the floor, it would be a good time to remove and properly clean the inside of the fuel tank, and put a sealer in there. It is a lot easier to remove the connections from the top of the tank while exposed, than it is to remove them when you are in the process of removing the tank from below.