I just got my December 2000 copy of Guns & Ammo and there is a large article in there about Marlin Lever guns.
On page 98 there is a small article about the 1894P in .44 Magnum!
According to the author Payton Miller the Garrett 310 Grain round will function in this gun!
The Barrel is 16 1/4 inches and the following muzzle velocity results are listed:
Federal Premium 240 Grain JHP 1,637 FPS
Hornady 300 Grain JHP 1,413 FPS
Garrett 310 Grain Hard Cast 1,496 FPS
The only problem with the Garrett round mentioned was that it shot way high.
Also there is a Thread about this subject in The Art Of The Rifle area.