The Kelsey-Hayes Disk Brake Swap Thread

64conv65hard said
"This Kelsey Hayes disk brake swap should work for any A-body with the 5x4 small bolt pattern wheel, EXCEPT those with 13 wheels". Why does this swap not work for cars with 13" rims? Are they built differently?

Rotors and calipers are too big and would hit the inside or top of the wheel.