I have ordered most of the parts to continue the build. After trying a hogburner propane torch to heat the solder up to remove that cap and failing I have concluded that the sheer mass of copper needing to be heated up exceeds my feeble efforts. The piece weighs 5 pounds... The "T" fitting is cast copper and very heavy and is acting as a heat sink to pull heat away from where its needed.
Because of this I'll lose about a pint of tank capacity, no big deal. It's still going to hold about a gallon.
I hope after bills I have enough money left to go to Andy's Supply and get what I need for a gas cap.