I think smoking the inside of the butt plate is a good start. Smoke it, be careful not to let it touch the butt as you install the screws. Just take it in until it touches and then a half or three quarters turn more. Take it off and have a good look-see, you might find out what's up. You said it was not totally true at the toe, that is an area to really check.
Preparing butt plates before you inlet it is a must for a good fit, you have to true the inside surfaces as you mentioned, and you have to be sure. When I have one inlet to the point it is almost done I only use a chisel to scrap the high spots shown by the blackening media. It takes time and you have to go slowly to get that no-gap result with those hard steel butt plates.
Good luck with your project.
dave