Wow, thanks for all the input, guys. Even from those of you who think the whole idea is stupid.
Dan, you mentioned that a small Siler won't feel like a first quality English lock when cocked, no matter what it looks like. I have never cocked a quality English pistoil, so I had no idea there was such a difference. Can you get this feel of a quality lock from modifying an L&R Manton? Or from Jim C's late Ketland?
Are there any tricks to thinning out the tumbler? If I remember correctly, the fly in the Manton is on the lockplate side, not on the bridle side as in the Silers, so turning from the bridle side seems to make the most sense. How much do you take off? BTW, I know what you mean about it making for a deep cavity. The first muzzle loader I ever built was a pistol kit from Golden Age with a Manton lock and a 13/16" straight barrel. Not a whole lot of wood left there. You can see the tang screw in the lock inlet. It did get me a judges choice ribbon at Dixon's, though.
I was going to stay away from the Rifle Shop because of the delivery time stories I have heard, and I did try Blackley's, but had trouble with their website. Maybe worth another look.
I did look at Jim's late Ketland for this pistol and the length was OK, it was just a little tall. Maybe I will take another look.
Right now though, it seems like Jeff's idea of contacting L&R to see if they will sell me a casting set is the best idea. I'll give them a call tomorrow.
Thanks for all the ideas, and keep them coming.
Hope you are all having a safe and enjoyable holiday weekend.
Dale H