I want a 375 Ruger and have always had pre-64 Winchesters, so that is my preferred baseline.
When I go to buy a complete pre-64, which would get modified into the 375 Rugr , what caliber pre-64 would be the best choice.
(Is it foolish to plan to use a pre-64?)
(Would it make more sense to get a new CRF Winney and have it converted?)
Once I have the base rifle, I will begin looking for a smith to do the work.

Whether you decide upon a pre-64 or a post 64 classic, it would be easiest to start with one of the standard mags (ie a 264 Win Mag, 7MM Rem Mag, 300 Win Mag or a 338 Win Mag). The bolt face and mag opening should already be properly dimensioned for the 375 Ruger.

I would suggest a caliber as close to the new caliber as possible to eliminate a lot of feeding issues. Work will still have to be done but less work will be done on a cartridge that is close

Built mine on a post 64 and it started life as a 6,5x55. I just opened up the bolt. Not a big job if you blueprint the action.

The boltface is really the only significant issue and even that's not a big deal. The actions are all the same except for some cuts for magnum calibers, which you won't need for the .375 Ruger, but if they were there it wouldn't hurt.