Ed,
You SURE did have a good day!
The downside of your success is that you didn't get the casting instructions with the mold. With most of my revolver bullet molds, sizing to fit the cylinder throat of interest is mandatory, and you'll have good results once you get the right sizer.
The WFN will not feed smoothly in a Marlin rifle unless seated quite deeply, which means a .350 nose length, and yours probably wears a .4 nose length, or even longer. One easy option is to use 44 special brass, but don't run out and buy some to try it, as you may not get enough powder in to get the 300 grains up to stability speed, which will require about 1800 fps.
The best bullet for Marlins is the LFN, with nose length of .450, and probably 280 grains will make most shooters happiest. A 300 grain will shoot with extreme accuracy when loaded to 1800 fps +, which is easy on the gun, but after one shoots a few rounds he'll begin to wonder if he's holding the wrong end of the rifle to his shoulder! I let my little wife shoot a 300 gr 1800 fps load at a deer once (which she missed much to my surprise). She was quiet for perhaps a half hour, then said. "Don't ever let me shoot that gun again." Her shoulder was black for a couple weeks. -- I'd have never let her shoot it had I realized it would bruise her. It takes about 10 shots to bruise my shoulder.
I told that story so no one reading this will get a bullet they don't like shooting. The 44 Marlin will kill deer and elk almost instantly with chest shots using the LFN at 1550 or the WFN at 1400 fps, so a weight should be selected that stabilizes well with velocities in that vicinity, if one doesn't like heavy recoil from the light gun.