Start at the starting load and work up and you'll have no problems using .310, .3105, .311, or .312" bullets in any Mosin. If the bullet's too big, it'll swage down in the throat of the barrel. It will of course increase pressure compared to a smaller bullet but starting loads are low enough that it doesn't matter. Just don't swap the bullet size without working the load up again.
Slugging the barrel could help if you're trying to squeeze the most accuracy out of the rifle but assuming you aren't trying to build a Mosin sniper or anything, you don't normally need to bother with slugging the bore unless you're looking to load cast bullets (or the bullets are tumbling and keyholeing on the target).
I actually load .308" flat base bullets in my Mosin sometimes (depending on what I have on hand) and it works great. No noticeable difference in accuracy. I've done the same with some tighter barrelled .303 British rifles as well. You could try the same in your Mosin. Worst case scenario the bore is too large and the bullets tumble and hit the target sideways but then you know for future reference.
For load data just use any data for the same bullet weight and construction type (copper jacketed/lead core, solid copper, frangible, cast, etc.). At starting loads a difference in bullet shape wont increase pressure enough to be dangerous. Again though, don't change the bullet without working up again.
