I've encountered that problem with engines that have had the rocker arms replaced with 1.6:1 ratio rockers, rather than the stock 1.5:1 ratio ones. It's necessary to relocate the rocker studs about 1/16" further away from the valve stems, and do some pretty sophisticated modifications to pushrod length and rocker geometry to make the 1.6 rockers work right. On anything other than an all-out race engine, the performance gain is not worth the effort.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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