According to ammoguide, the 375H&H can be rechambered to 375 H&H Flanged magnum, as every dimension of the flanged case is the same as or slightly larger than the belted case. The reason I am considering this is that I want to build this cartridge on a Farquharson action and the Ruger conversion would be a good trial run. I would like to know if someone has done this conversion and can share the experience. Besides the extractor on the Ruger,is there anything else that must be changed?

According to CIP chamber dimensions, the diameter of the flanged case is .518" just above the rim. This is smaller than the nominal .532" diameter of the belt of the rimless/belted case. So, the flanged case will not be supported for the .220" height of the belt recess and bulged cases are likely to result.
No. 1 ejectors are easy to modify. No other modifications needed, but the flanged reamer will NOT clean up the belt recess.

The #3 in 375 Win might work as a project. I've got one that has been chambered to 375 2.5" Flanged.