[atijohn]

here's the poop!
the 918310 has a chevy bolt circle 3-.3200.
the 916581 is .375 long and converts to ford 3-3.300 circle.
shorten the spacer only if you want to run an oem ford pulley.
just like a wheel adapter
it's done like that because chevy trigger wheels, mandrels, and lower pulleys are more readily available and less expensive.