I didn't add the shim to mine and I'm running 30%, I'm not sure if you are suppsoed to, but it does run fine without it,
I run MC59's in pretty much any motor, it also depends what fuel you ar running, 30% generally likes a hotter plug I think? I've also found that big block engines run better with a hot plug too, I'm also in Canada too tghouh, so it doens't get as hot here as say Arizona or somewhere like that,