Excuse my engine building ignorance upfront. I have a stock freshly rebuilt 383 (factory stock rocker arms) with a new stock cam that is only pulling 12 inches of vacuum. An engine guy told me that if the heads and/or deck have been milled too much in the past, it could be preloading the lifters too much, resulting in the loss of vacuum. The fixes I assume would be adjustable rockers or custom length pushrods, right? Forgetting any other possible causes for low vacuum, does this explanation sound plausible to you guys?