Chuck, you are correct about more wood is removed between the side plates for a HOT. At an auction I won a Field Grade 16 ga with a HOT and ejectors for a very good price. I knew the stock was cut, and I was fortunate to find another from an Ideal grade but it did not come from a HOT gun. It took longer to set the stock up in my mortising machine than to cut it out. Luckily a 3/8" mortising chisel fit perfect.
I am not sure if the Miller takes up as much room or not, as the only one I have with a Miller selective trigger was converted from a HOT.
Below is a picture of the Miller in my Ideal Grade, notice that the housing for the triggers is the same as a double trigger gun.

Picture of a HOT, notice that the housing is the same for the triggers.

With a HOT trigger on a Featherweight frame, not only does more of the precious wood have to be removed, but metal has to be taken out of the top tang for the spur lever to rise on the firing of the second shot. That is why the safety is manual with a HOT on a FW frame. On a Regular frame, no metal has to be remove, but the safety slide bar cannot be used, and a special one has to be made.

