I think You'll Regret cutting the barrel that short the fireball and muzzle blast will be nutty crazy. If ya want a short one get a M44.
Or an M38 or a 91/59. The M44 has a bayonet, the M38 and the 91/59 don't. But 6' long fireballs (in broad daylight, no less) are fun.
Keep in mind, too, that the Mosin carbines, while shorter and a bit handier, are still nearly as heavy as the full length rifles.
If you're going to hack on the gun anyway, consider cutting an inch or two off of the stock and replacing that length with a recoil pad. If you're going to defile the thing by cutting down the barrel, bending the bolt and drilling and tapping for a scope, you might as well make it slightly less unpleasant to get smacked by while you're at it. Or, rather than hacking the stock, you might consider
one of these, but hacking the stock and installing a pad will be cheaper.