8.75 Spring perch Relocation Measurements

Correct, that is what i figured as well but wanted to make sure. So in all reality, i could take the stock measurement from the outside of the housing to the center of the mounting surface and move it in 1/2" on each side. Or like you said, center to center is now 42" vs 43".
Yes, if you have the offset kit he sells for 175 you can just have the shop move the perches 1/2" inward each side. It doesn’t need to be all complicated by measuring from the outside of the housing etc, just move em over 1/2" from where they are/the stock location. Make sure they keep the same pinion angle and or reference it to just make sure they recognize that.