For your fuel, compression, cam and quench situation, the motor responded bestTO AN OBSERVED 30 degrees. So trust the time slip. But if you change fuel, or timing light, or cam timing, then you will need to retest the timing it wants. The next time you have it apart, recheck tdc marks and cam timing. That way you can be certain of the observed timing
 By all means, run it where it likes it. The timeslip doesn't lie. I'm very surprised if that motor didn't want 36-40 degrees. Like said, something may be off giving you an inaccurate reading.
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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