IMP .30/30 is always good idea. The brass lasts forever but with normal .30/30 bullets, but expansion could become too great, limiting penetration with lighter .30/30 bullets. Certainly, on deer or black bear, a 170gr. moving 2,400fps will make it a better deer bullet. I would not use 150's in the ImP case.

9.3's idea of shortened 220gr. is a good one and would elevate the .30/30IMP to being a good elk and moose timber round. Shortened to 190 to 200gr. with a nice flat nose, and running 2,200fps to 2,300fps, they'd be much better than the .30/40 Krag.

The Marlin should handle the IMP .30/30 very well but may not make it into a .307, of course - but very close. Watch the headspace and make it perfect and it will amaze you on the chronograph & on game. Scoped with a straight 4X or a 1-5X, they're very accurate and fast, although make the shot right and only one is needed.