I would clean the tank out good with maybe a light muratic acid. follow that with a good rinse and let it dry. Then use the tank sealer. I've never used redkote but the sealers I've used advertise they can stop small pinholes. I'd let the sealer permeate that seam real good with nothing holding it back on the outsid. If it leaks too fast, use some tape or matting to hold it. If you want to use JB after the fact, it will be fine but I wouldn't use it first. My experience has been if the gas can get to the backside of the JB it wil usually come loose and leak. They make a 2 part putty they claim will seal in gas but I've never had much luck with it either. I wouldn't think that tank would be real hard to find.
muratic acid works great for removing rust BUT it will need to be sealed within minutes of flushing the acid out of the tank.If you don't seal the inside of the tank after the metal is oxidised by the Muratic acid it will rust 10x worse than the tank was before you cleaned it within a couple of days.
I ruined a motorcycle tank that I was trying to clean out with acid.I thought I had a can of gas tank liner but it had dried up and by the time I had the time to go get some (almost a week) the rust had returned with a vengeance.Rust was so thick that it was literately flaking.
If you solder the seam you are going to have to solder the whole way around.The heat will melt the sealant that they use on the seem.
You could heat it and burn the sealant out of the seem and take the tank apart and clean it then Solder the seem back up then coat the tank.That way you know if there are any thin spots in the bottom and you could solder those up before you put the sealer in.