Well now, that sounds like a mighty ambitious project!
As I recall seeing somewhere years ago, the old English & Belgian double-gun makers didn't so much use jigs, as they did wire.
In practice, the mating parts of all the surfaces were cleaned and acid flux applied. Then all the parts were carefully wired tightly together with soft iron stove-pipe wire, and aligned with little iron wedges driven under the wire wraps.
The idea being that the soft iron wire & wedges would expand at the same rate as the gun parts when heated, and thus stay tight.
Then solder was placed where it needed to be, and the whole mess was put in a gas furnace to heat the assembly up until the solder flowed into the joints.
Sounds good in theory, especially if you were an old English or Belgian gun maker with a lifetime of training and experience on where & what to wire & wedge together, and where to place the solder before the Bake-Off!
About all I can say, other then that, is Good Luck!
You will need it!
rcmodel
