1) IS the hub configured w/the SBP ?
2) Is the hub fitted with 1/2" or 7/16" studs?
3) There is no "just removing 3/4" from the center" of a steel rim. The rolled hub ring will be completely removed, and one from a wheel that fits welded in, or fabbed from scratch.
I certainly wouldn't do it for any less. Yes, if One were to just chuck up the rim & cut the center out w/o doing this, the integrity of the rim is compromised. The problem is if You
do the math, 4"-3"=1" & /2= .5" from the center of the stud hole to the new ID, w/a 1/2" hole that only leaves 1/4" of meat & no room for a lip & barely a socket over the nuts.
4) Every wheel is different, I'm sure calipers clear certain 15" rims, but not all....of course the spacer sounds like they knew they were tight & needed.
5) What ride is this & what is the intended usage for it? You may have to sw. to LBP, but why they would even sell those this way....................