You can put the wrong width belt, but I would not recommend it.
I do not think you need a double belt PS pump.
I would think that the pulley on a PS pump can be changed so you do not have to run the wrong size belt.
I do not know but did that vintage have one belt for the WP, PS and Gen, or a two groove pulley on the crank.
I would try to duplicate the original pulley lay out. Jeffrey
