I am also working on some +P loads. In particular:
180 gr Speer - 2350 fps
200 gr FTX - 2250 fps
220 gr Speer - 2200 fps.
These should all keep the pressures in the 40,000 range. I don't think you should push a Marlin much over 42,000, but I'll also shoot these in a 760 Remington which should handle a lot higher (50,000) pressures.