questions on rear fitment

[721302]

i have just about got my 18's on my 72 super but i think i might need a spacer in the rear. i trimmed the bumpstops and put a flat headed bolt in the furthest out spring plate bolt but they still seam very close to the bumpstops and spring plates. I can just get my finger between the tire and bumpstop. how much clearance is recomended between wheels/tires to suspension componets(front or rear)? I have 18 x7 wheels, et40 with 225/40/18 tires on them if that helps answer anything.

[zen]

7ET40 shouldn't be an issue based on what i know of others on here running them. clearance required is relatively minimal although i can't state a number.