spark plug heat range questions.

well today i purchased 8 brand new autolite 3923 plugs, i have heard nothing but good things about them in the 5.2 and 5.9 magnums, they are 1 step colder then stock heat range plugs. I was running the ngk v-power plugs the zfr5f11's and i was told the zfr5f11's are stock heat range, so i wanted to go to the ngk v-power zfr6f11's but the two local parts stores i have in this town didnt have the ngk zfr6f11's. So i decided screw it i will run the 3923 autolites, even tho i have no past expirence with aoutlite plugs? but since i had read good things about the autolite 3923's in the magnum engines.
So i fired it up and then i switcch my vaccum advance from ported vaccum to manifold vaccum and it still was thuddin out the exhaust, a colder spark plug did not cure that problem. So i put my vaccum advance back on ported vaccum.
Currently running 14 degrees of initial timing 32 degrees total timing and i have my vaccum advance set to add 12 degrees of timing at 18 inchees of vaccum. So i think i will leave the vaccum advance on ported vaccum and stop trying to get this engine to run right with the vaccum advance on manifold vaccum, aparently 14 degrees of initial timing i plenty for this 360 magnum of mine. It still runs good, just as good as when the vaccum advance is on manifold vaccum, the only thing i liked more about my vaccum advance being ran on manifold vaccum is that my engine was cooler while at an idle.