My home inspection of Speedmaster heads

From the picture in 610 the plug needs to be about .060 deeper at the most.

You are correct that you sone want any part of the shell in the chamber. The face of the shell should be flush with the chamber.
So ideally, machine the bores to depths needed and then index the plugs so the strap is close to long side, about as perfect as you’d get things. Maybe a new procedure for you machinists out there, make a fortune!