549,
to fit the 375 HH Flanged Magnum into a P14 ( probably the only action for this conversion ) takes a lot of modifications to the action.
I converted a P14 to 9.3x74R ( slightly longer than the 375 ) and it involves a lot of metal removal, new mag box, etc, etc, and on and on....
There are only two of these sorts of conversion that I know of, mine and Young Finn's in South Africa...his being on a slant box Rigby action.
To pay someone to do this conversion would be extremely costly, just for the metal work alone....I am not trying to discourage you, just the facts.
Roger