.450 Marlin need reduced load for 300 gr fn

You could just use 45-70 trapdoor load data. The 450 has a slightly smaller capacity, so pressure and velocity will be a little higher with the 450, but the trapdoor data will still be safe.
Andy

I use 20.0 grains of 2400 with .5 grain of poly fill with my RD 425 mold and it's a real pleasure to shoot. Plenty accurate to 50 yards, approximately 1250 fps from my 18" guide gun barrel.

It's a GC design bullet, but I don't need to use a GC with this bullet/powder combo.

Try, IMR-4198 with your 300 gr. cast bullet: 25 grains should be a good load.

You might also want to try Accurate Arms XMP5744 for reduced loads in the .450. That powder is not position sensitive and I get sterling accuracy with it.