spark plug heat range questions.

Got a few questions regarding heat ranges of spark plugs. Can running a spark plug with too high of a heat range cause idle problems when running a high initial timing and running vaccum advance off of manifold vaccum. and while running premium 91 octane fuel?
You see, i have my initial timing set at 14 degrees wich lands my total timing at 32 degrees, engine is a 300hp crate 360 magnum, compression ratio is 9:0.1 and i run premium 91 octane fuel in it and i live 4000 feet above sea level.
Whenever i try to run my vaccum advance off of manifold vaccum it will run fantastic, but everytime the engine is at an idle with the vaccum advance on manifold vaccum there is occasionally a thud thud thud out the exhaust while at idle, wich almost sounds like a miss. It never did occur to me untill today that maybe i need to go down a heat range or two, i'm thinking maybe with all the initial timing i'm running at an idle with the vaccum advance on manifold vaccum and along with the high elavation i live inn and the premium 91 octane fuel that maybe i could get this thudding or miss to go away while at an idle with a colder spark plug.
Is it possible that maybe the plugs i am running are too hot and that they are causing a miss at idle?
I am currently running Ngk v-power spark plugs the zfr5f-11's, i have been told this is the stock heat range for a 360 magnum. And that the next step colder plug would be the zfr6f-11. Or i could get away from the ngk's and run the autolite copper core plugs and the stock heat range for the copper core autolites is 3924 and the next step colder is the 3923's.
Is it worth trying a 1 step colder plug and seeing if this issue goes away?
Currently i have my vaccum advance running off of th ported vaccum source on the carburetor , but i have een told this is wrong and that it needs to be on manifold vaccum. according to the gentelmen that built my distributer.