@Rule 303
Sorry to say but your gunsmith is pissing in your pocket and telling you it's raining.
The P14 bolt face is set for the rimmed 303 British which has an identical size to magnum cases from 257 Weatherby to 458wm.. It may need to be opened up a bit for the big magnums tho.
The bolt face of the M17 is standard 06 size but can be opened up to suit magnum cartridges.
Why would anyone stuff around modifying the mag box to suit the 375H&H when you can buy one for $75 if the internet.
The rear of the action is easy opened up to make it suitable for bigger cartridges and modified the bolt stop to suit. The ears are easy to mill of and can even be cut off with a hacksaw then milled to finish easily.
The issue is filling the gap in the top of the action where the rear apature sight sits.. This is usually done by making a block to go in it then milling the rear of the action to the profile you want.
Dang you could even do a single square bridge Mauser profile if you wanted.
Yes you can go to a cock on opening but this cause issues with the sear so why not leave it as is, it works and is actually quicker to operate as cock on closing when you get used to it..
If the gunsmith is knowledgeable then working in an Enfield is no different to any other military Mauser style rifle. Problem is some Smith's want you to believe it's harder and therefore more expensive. Bullshit.
For example straightening a ninety degrees bolt handle is the same as changing the dog leg. Same amount of work in both but for some unknown reason the charge is higher for the Enfield.
I could keep going but I've made my point
Bob