Marlin currently makes the .38-55 Cowboy Edition, a variant of the 336 action. Sorta expensive, but it already has the octagon barrel, case hardened receiver, etc.
After looking over my .38-55 M1893 (prececessor of the M336) I see no reason why your conversion would be a problem. The 336 was designed of course for .30-30 Winchester, but the parent case is the .38-55.
Let us know how it shoots!
John