The guy who modified his Delta SA180 who published the pdf on how he did it used two 1/2" bore step pulleys bored out to fit the shafts on his machine.

I have a Delta 31-340 which looks identical to the SA180. My motor has a 5/8 inch shaft, compared to the SA180 in the .pdf which the guy says is 16 mm. So I had no problem getting a 5/8 inch bore step pulley to fit that.

My driven shaft is 15 mm, just like the SA180 though. I suggest that a 3/4" bore step pulley could be used if a 15mm I.D. 19 mm O.D. bronze sleeve bearing is fitted to the shaft as an adapter first. 19 mm is .748". One wrap of .001" shim would take up all remaining space. What I actually did rather than wait for a 15 mm I.D. 19 mm O.D. bearing to arrive was to take the 1/2" I.D. 3/4" O.D. bronze sleeve bearing I had and modify it. I hacksawed a slit lengthwise then pressed it onto the 15 mm shaft which expanded its O.D. to around .78". Then I rotated the shaft while holding a file against the bronze until it was reduced to .75" Then a .75" bore step pulley fitted perfectly.

I got my set of 4 speed step pulleys from a dead lathe on craigslist. One was 5/8 inch bore and the other was 3/4".