Easy to do with machined pulley's (quality straight edge off top pulley, 4 measurements off bottom pulley, move other end of track shaft to square up). Hard to do with stock pulleys. Even being out a few thou. with center to center on the shaft ends on either side will have the belt walking.
As Loudhandle said, the width and rigidity of the belt requires tighter tolerance.
There are some things to check. I did some long winded posts in '13 '14 and '15 describing as best I could in words if you want some reading.
First thing is if the QD plate was glued on square to the tunnel (see if the jack shaft shoulder and plate have clearance before torquing on the top pully, push in on the top and see how much it flexes until touching, shimming if needed).
The other is the parallel shafts you are thinking about. You can adjust this by moving the opposite side of the track shaft (loosen the 3 bolts and jiggle around is sometimes enough, sometimes requires a bit of carving on the tunnel holes, once centered a couple of well placed rivets will be your final alignment spec for future work).
First thing is to find out is if the belt walks over to the other side of the pulley when run in reverse (to eliminate a funny worn belt possibility). If that is the case then you can fix with the above.
You can also use this as a-long-way-around alignment checking tool if needed.