Which wheel spacer to use

I have SVT 1999 cobra wheels 30mm offset, 5.72" backsp on a '95 GT with stock rear brakes, stock axles. What size wheel spacer should I use to make the rear wheels just flush with the fender.
Do you recommend hubcentric (slip ons) or lugcentric (bolt ons)? Are longer studs needed?

Thanks for all your help.