RE: Leveling crosshairs
For me the issue is straight up & down so you don't cant on the long shots. I would try reading an axis on the plumb line, from the top center of the scope to the bottom center of the stock. If it is real important, braces and sleds are out there for not too much money.
Canting a rifle was an issue with the M16 at 500 meters because the iron sights were relatively high in relation to the bore. We did not do anything fancy, but we kept straight up and down as best as we could call it.