Problem with lining up the outside edges of the pulleys is, depending on the process used to manufacture the pulley, generally factory does not bother with machining or otherwise refining the outside lip or edge of the pulley, so there can be thickness and/or other dimensional variations between the center of the Vee of one pulley from the other, which would lead to mis-alignment I know of only a few specific applications where the outside lip of the pulley is machined to specific tolerances in relation to center of the vee, and those generally are very very expensive parts (the ones I have seen in person are used on outdoor power equipment - and usually the outer surface of pulley is used to drive something, or machined to tolerance for a brake, etc. Then there's the Polaris CVT/ Reeves drive used on many ATV's & snowmobiles which have an alignment tool, so their unique pulleys are machined to specific tolerances for alignment)
For drive belt alignment, you actually want the center of the Vee notch to line up, which might be accomplished with a thin steel rule laid flat along the vee notch with the edge of rule in bottom of Vee , then you might line up the belt surfaces (assuming the rule itself was perfectly true and flat and not allowed to bend or flex) Or, you could get a known straight edge (wood or metal) and machine one edge to the same Vee angle as the drive pulleys...
