Had this issue with my 67 coronet w/ a 440.. There are all sorts of differences from year to year and pump to pump..

My lower two groove pulley ( on the crank/damper) had rusted in two when I was rebuilding it.. NONE of the pulleys lined up. Seems as if this was a 1 yr only thing? and was appx 1/8" shorter in height than all of the other pulleys in the world.. Randy, 1wildRT helped me out with one he had...

Same with the PS pump. Lost mine somehow ( car was apart for 18 years) and the TRW pump it came with had two different sizes ( small block/bigblock?? ) not only in pump diameter but pulley distance from the main body.. Only way I finally got mine to work was going through a barrel of old pumps at Wildcat Automotive with a tape measure.
I can't imagine tryng to upgrade or change to a different model pump after what I went through.....