440 detonation issue

You could try limiting the mech. advance by welding up the slots inside the dizzy. When does the dizzy reach full mech. advance? If its fairly early, you could also try one heavier spring inside.

...or it may be all carboned up. I'd try the "drip water down the carb while revving the snot out of it" method.

..or the plugs are too hot.