Shutting off the fuel pump won't do anything unless it's a fuel-injected engine. You've still got fuel in the bowls. To me, I think the reason retarded initial timing could cause dieseling is because you've got to have the throttle blades cracked open more when the timing is retarded. I'd try to get the idle down as far as I could first, like maybe 750......and as much initial advance as it'll handle.
