Re: Tank petcock
If you absolutely want or need it to sit flat, I have no idea.
If you could tilt it backwards even a little bit, you should be getting most of the fuel.
Really, without a welder; there is not much you can do about it. Do you have the ability braze? I would suggest solder but that bond is not strong enough for this type of work. Without either of these abilities, I would suggest tilting the tank unless you are going for a rat-rod/Frankenstein look.
Cap the OG petcock with a bolt and a bunch of epoxy, cleanly though; don't get messy. Drill a hole in the location you want it, big enough to fit a brass 1/4" hose end from the plumbing section of a Lowe's or Home Depot. Yet more epoxy, and you should be home free!