Read your post about a custom lever gun. I too have had that itch, but after talking CP Donelley at Siskoui Gun works, my interest was tempered. There is no cheap way to do it.
If you want to have a 375 win, then just buy the Marlin Cowboy in 38-55. You then can have the best of both worlds since the 375 will work in the Marlin lever gun.
Strickly money wise, you be better off buying the factory gun and having someone do an action job on rather than spending a bunch, I mean a BIG bunch of money, on rebarreling a winchester or marlin to another caliber which then will have a limited resale value relative to your investment.
Given all the above and you still have an itch for a custom rifle, then go for it if you have the bucks.
Snakeoil Willie