I BELIEVE the PSI wheels are 1/2" wide and Beall is a little over an inch.
As far as how they are held, you CAN use a threaded rod two feet long and separate the wheels with PVC and nuts - then use any wheels you find at your local "cheapy" store. Has worked for me.
I also have the Beall wheels, they are nicer - but it all depends on how many pens you plan to make. My Beall produced about 1000 pens, so on a "cost-per" basis, it wasn't too bad.
You gonna make 1000 pens in this lifetime??? IF so, spend $70 (I think that was the price).
FWIW