Joe, did you file the bolster of the lock plate prior to or during inletting? You need the full thickness of the plate's bolster to maintain the width through the lock area, and subsequently, the wrist. If you did file it, you should likely add steel by soldering on a plate. But if you filed or ground your frizzen you may need a new frizzen. What is the barrel's diameter across the flats? Can you take a picture looking down on the breech, showing the lock plate installed. You may have been preoccupied with trying to get a taper into the lock area, and that will have left you too thin at the tail end near the wrist.
Don's S. Hawken has a 40/1000" taper. That's a little over 1/64" on each side. It may even have been unintentional - how would we know? But it is insignificant, and practically invisible to the human eye. If this was my project, I would strive to get that lock plate parallel to the barrel and out to the periphery of that mortise. All doable.