You sure it's not ignition related? I'd be swapping out coil or ignition box before I pulled a valve spring unless you find broken ones (could be product of valve float).  I'd rule out the ignition system first.  I also agree that those spring pressures seem light for a heavy hemi valvetrain.
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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