My understanding is that you can buy .357 mag ammo, from places like Buffalo Bore and Garrett, that are loaded way up, with ballistics that closely approach the standard .35 Remington, for use in the Marlin 1894 rifles. Of course, the serounds are expensive, but if you just use them to sight in and hunt, then it would be a much better way to go than trying to convert a Marlin to .357 Max.
Just a thought.
Mannyrock