A keyway next to a keyway will change the timing waaaaaay too much.
A keyway next to a keyway will probably alter the timing by 30 degrees.
You need a change of more like 2 or 3 or 4 or 5 degrees.
That is why the "slotted holes in the mounting plate" is the way to go.
You can alter it a little bit any time you want... by however much you want.
In either direction.... no expensive, unreversible, time consuming, machining... just loosen the screws and rotate it and tighten the screws.
I expect the machinist will tell you the same... (if he knows anything about engines).