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.
Actually, the 12 ga's case head area is only slightly more than twice that of the .45-70 (.616 sq.in. vs .290 sq. in.)
The .45-70 max chamber pressure is 28K, not 25.
Comparing max 12 ga thrust load against the .45-70, we get:
12 ga - 6714.4 lbs.
.45-70 - 8120 lbs.
I also believe that the action can easily contain that pressure
but as I stated earlier, I didn't want a 20 pound "me too" gun.
