When you're doing a performance engine with an aftermarket camshaft or just want to get all you can out of a stocker, it's best to degree in the cam. Replacement head gaskets can vary in thickness by almost .020", and deck height affects cam timing.
If you want to be fairly precise, you can do this. First, since you're going to have the head off anyway, get a cheap dial indicator and set it up on top of a piston to find TDC accurately, then correct your alternator timing marks if necessary. If you can find a small clear plastic protractor about the same diameter as your alternator rotor with enough material for you to drill a hole the diameter of the crankshaft, modify it, remove the rotor nut, and snug it down against the rotor; with the alternator marks at TDC, line up the 0 mark with them. If you can't find a protractor that will work, copy the degree wheel from 650 Central, size it to the diameter of your alternator rotor, copy it to transparent film at a copy shop, cut a piece of plexiglass to the same diameter, and stick the film down on it. Remove the right engine cover so that you can turn the crank from that side and avoid disturbing the degree wheel.
I don't know what year you have, but for specs on 256 engines see the Clymer manual, for 447's see the factory manual at
www.biker.net . Set your valves to the specified inspection clearance (not the same as running clearance). All OEM XS650 camshafts specify valve opening and closing events at 0 lift; that is, the degree numbers refer to the point at which the valves just begin to open and close. If you don't want to fabricate a holder to set up the dial indicator on the valve collars, insert a piece of cellophane between the tappet and the valve stem, rotate the engine, find the point at which the tappet and valve just release the cellophane (valve closure) or grab it (valve opening), and read the degrees between your stator mark and the degree wheel 0 mark. Do this several times to make sure your results repeat.