First & foremost let me say You should definitely have built what you wanted. As a retired machinist I truly admire the work you have done here.
I do however want to make one comment about the chamber pressure of the 12ga vs the .45-70. Chamber pressure is of course given in PSI or pounds per square inch. The chamber walls of the .45-70 would certainly have been thick enough to handle the increased pressure. As to the thrust on the action the 12ga has about 2˝ times the area of the .45-70 at its head. Therefore a the thrust from a 10K load in 12ga would be about equaled by a 25k load in the .45-70.
Ideally though to build a .45-70 I would want to drop to a 20ga frame to get the narrower & shorter standing breech. I think you made the right choice for the 12ga frame.
