I've heard that the wheel well opening is wider on 78+ vans. I havent actually measured for myself, but it's a common rumor and said on most websites selling flares and such.
Ok here is the deal.some where in the early 70's chevy DID change the size of the wheel opening.Chevy made them larger.
Its proably the same year they changed the bolt pattern from 5x4 3/4 to 5x5.
I have a set of flares from a 76 chevy that fits my 93 GMC.so the change was pre 76 I do know that for sure.......
...........Wookee
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