plugs+thermostat are not going to reduce your KR by 4 degrees, assuming that is the actual unit of measurement you were given. How much timing did they pull from your WOT to allow you to run relatively safely on the 3.4? You will make more power running max WOT timing (~20 degrees) with a bigger pulley, versus reduced WOT timing and a smaller pulley. In either case you want 0 KR.
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'99 GTP, L36 bottom, XP cam, Headers, 3.29 gears, FWI, Powrtuner