I'd go ahead and buy the gun you want. There is a bullet out there that will do what you want it to do. There is no certainty that the 300s will not stabilize in a 1-38" twist. I had a Winchester 94 trapper that handled them just fine. In those days I preferred the now-discontinued Speer, but it shot the Hornady very well, too. If your gun won't handle them .. and you won't know for sure 'til you try .. then try Speer's 270 grain flat point. I think Hornady's 265 grain flat point interlock will seat to an OAL that will fit through a marlin but I'd check on that before buying. Hell for stout bullet and a good choice if it will.
There is nothing wrong with a 240 for deer. My preference would be one of the flat points instead of a hollow point. Might also consider Barnes X bullets in the lever action.
