The seat frame is bolted on each side. Remove the bolts, raise the seat frame to clear the assembly. You will still have the welded piece on the floor, but the seat frame itself will come out allowing access to the tank.
I second getting the tank boiled out or a new one. I ended up having to get a new tank as mine was so bad the local radiator shop couldn't clean it. If you end up getting a new tank, be sure to ask about the threads on new tank. My new one came with metric threads.
