Welcome to AF, assuming an accurate gage, 60 is too low, 65 is required. See if the fp holds for a few minutes or drops off quickly as the Vortec injector units have been troublesome. Yes the dist gear can wear also on these trucks, usually will set a different code though. That could account for the timing. Check this link for the p0151 parameters.
http://service.gm.com/gmspo/mode6/pd...t57R_LD_yE.pdf