Re: Copper Tubing Fuel Tank
I would use the propane tank as suggested. Don't use a gas which is too hot you'll wreck the copper. 2door's advice to wiggle it when the solder is molten will do the trick. Then when you have it apart and cooled you can use steel wool to clean it up where the solder was. Emory cloth is good for that, too.
I also had considered the copper pipe for an in-frame tank a year ago and like you could not believe what it costs and revised plans in a hurry. Rest assured, yours will be one of the few if not only ones simply due to the cost of materials. Searching a salvage yard is a great idea. Good luck.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...