I have run into this type of boo-boo before. Because of the distance relationship between the tang, touch-hole and lock placement, installing another breech plug with the touch hole farther forward causes more modifications to be made. That hole has to be exactly where it is or a bunch of changes will have to be made.
I have in the past had to increase the thread depth of the breech to about 1-1/2 inches and made a new breech plug from a 5/8 x18 grade 8 bolt and sawed and filed a new breech hook or weld on the original tang. Then drill a communication hole down the center of the breech plug and a new drum/touch hole through the existing one to intersect. Sort of a homemade chambered breech but much safer and easier to clean. A lot of hand work though, but all the dimensions are unaltered. The original builder on this rifle didn't position the barrel far enough to the rear. That is the way I do it when using a drum on a 13/16ths 45 barrel or a 7/8ths inch 50 caliber barrel. I want full engagement of the drum threads on a thin wall barrel and the only way short of a patent breech is to make a long breech plug with a firing chamber.
Bob