A few random thoughts about scoping a Mosin, other than what's already mentioned...
To add an original PU scope mount, holes must be drilled in the LH side of the receiver, and a relief channel must be cut in the LH side of the stock. Some 91/30s known as "ex-snipers" have the required holes already drilled in the receiver, but these rifles are somewhat uncommon, and most importers are now savvy enough to recognize them and charge extra.
A PU scope does not adjust like modern Western scope; when you adjust its zero, the FOV remains fixed and the crosshairs move. Yes, this means that you may wind up with way-off-center crosshairs if the mount is a bit crooked.
The straight bolt handle won't clear the high-mounted PU scope. The reason for the high offset scope position on Eastern Bloc sniper rifles is to allow the iron sights to be used as backup with the scope still mounted,
not to clear the bolt handle.
The bolt handle on a Mosin-Nagant is attached to the bolt body, which has
no effect whatsoever on headspace because it's a totally separate from the bolt head. If you want a turned-down bolt handle and have a numbers-matching straight-handle bolt body, I recommend replacing the entire bolt body rather than hacking your original one.
The common ATI Mosin-Nagant scope mount kit includes a bolt-on turned-down bolt handle, but installing it requires hacking off the original bolt handle, grinding the surface flat, and drilling two precisely-located holes. I would
not recommend trying this unless you have access to power cutting tools and a drill press because the steel is hard enough to make this task a major PITA if you use hand tools. Buying an aftermarket bolt body is far easier.
Speaking of aftermarket bolt bodies, the original Soviet turned-down-handle bolt body requires a vertical relief cut in the stock. I'd recommend using an aftermarket bolt body that doesn't require a relief cut; they're usually cheaper anyway.