sometimes the dieseling issue is because the throttle plates are open after the key is shut off. this can be especially true for a high perf engine that has trouble idling because we automatically speed up the idle, this allows the engine to draw in air/fuel mixture until the engine stops rotating. if the engine is hot when shut off this mixture can self ignite in the chamber from a hot plug or some carbon build up, causing the dieseling or "run on" and that pinging sound. on newer carbed engines they used the old fashioned idle speed adjuster screw to adjust the throttle plates so they would close as much as possible without actually jamming inside the bores, then an electric solenoid was used to bring the throttle lever on the carb up to a suitable idle speed when the key was on to energize the solenoid. the solenoid had an adjustable plunger to adjust the length and so adjust the throttle plate angle-idle speed. with the key on the idle was set but as soon as the key was off the throttle plates snapped shut so no more air could pass and bring a bit of fuel with it. you may also check for a vac leak under the carb or an intake manifold gasket. the latter can be especially true with heads that have been planed down several times because the angle and distance between the head gasket surfaces and the intake gasket surface changes when the heads are planed. check anything connected to your intake system, hoses, brake booster vacuum advance operation, centrifugal advance mechanism etc. check the age and quality of the fuel as well because lower octane fuel explodes easier than higher octane. check where you buy fuel as well, some places have the same octane level advertised but they start off with poor quality of fuel and add alcohol and other things to bring the octane level up. some of these newer fuels also eat up fuel system components on older systems. you could start with checking the entire system, a valve adjustment check, compression test, spark plug check, complete ignition system check, fuel pressure and flow check, carb settings, proper pcv valve, vacuum hoses and fittings etc. just to be sure the whole system is functioning properly. they are all related.
here is a quick article that may help you do a few more checks, worth a read anyway.
http://www.badasscars.com/index.cfm/...d=94/prd94.htm