Is the case capacity of the .375 Win much greater than the 35 Rem? If not, you may just be better off loading your .35 Remington 336 Marlin to it's full potential. I've been loading my 336C and 336SC (both 20" barrels) with the now discontinued Alliant Reloder 12 powder and velocities with a 200gr HDY/REM RN are right at 2300fps with superb accuracy. This load makes for one heck of a deer thumper and larger game would not be out of the equation either. Alliant Reloder 12 has been my go to powder in the .35 Remington, .300 Savage and 30-30 reloads. Luckily I bought 19 pounds of Reloader 12 (all Lot # 012) when Alliant announced they were discontinuing it back in December 1998. It is also an excellent powder in my three .358 Winchesters I have. Lately I've been loading Ramshot TAC in the .358's and can push a 200gr HDY pointed bullet in excess of 2670fps and the big 250gr HDY RN in excess of 2400fps with no pressure issues at all. The velocities achieved in the .358's using TAC, actually exceeds the factory 200gr (2638fps) and 250gr (2378fps) velocities of my .35 Whelens. Just two cents worth from an old .35 caliber fan!!!!