I think you are over estimating your velocity possible with that load by about 400-500 FPS. I have chrono-ed my 42" .45 barrel with 70 grains of 3F at just a little over 1500.
That seems kinda slow for 70 grains from a .45 with a tightly patched ball.
I chronographed my flintlock .45, 42" GM barrel with both 3F and 2F powders. When using LHV lube, 75gr. 3F GOEX produced 2,200fps with a .445" ball and .022" patch.
85gr. 2f duplicated that load's velocity, same ball and patch.
I got the same velocities in my .40, using 65gr. 3F and 75gr. 2F all GOEX powder.
Joc- I did exactly that with my wife's Seneca .36, replacing the barrel with a 36" .45 GM barrel. The girl's have been using 55gr. 3F GOEX for shooting the trail and it works just fine, with
water based lube.
I found a Hawken lock from Track just about perfectly fit the TC's lock mortise, but of course, I had to file the lock plate a bit to make it fit and inlet the parts - a small job.
I also preferred a Bedford county hammer to the Hawken hammer, which was too big and didn't fit the geometry of the gun as well as the Bedford hammer.
This is my daughter shooting that rifle. Pls excuse the steel rod. It was a weight adding feature for shooting the trail.
Years ago, I changed the entry pipe to a cast babbit chunk. The forend could have used another key, to balance it out.