My first thought is that is a lot of compression for iron heads and low octane fuel, You will not be able to run an optimum advance with that fuel without detonation. Don't be fooled, detonation (spark knock, preignition) is dangerous to your engine and can even occur if you can't hear it. It's the equivallent of taking a hammer and beating on your piston tops.
Magnums are fast burn heads and don't need a lot of advance, on the order of 32 to 35 degrees total.
I don't know what those plugs are equivallent to but you should be running something on the order of a N7Y or C63C Champion. You need higher octane fuel. At least 91-93
Cylinder temperature and pressure are controlled by many factors including the cooling system and camshaft timing, duration and overlap, spark plug heat range, compression ratio and timing curve.
My advise would be to first check your carb butterfly opening at idle speed. Remove the carb and check to see if the butterflies are past the transfer slots in throttle plate. You are probably opening the butterflies to far to keep it running and now the carb isn't running off the idle circuit, instead it's running off the primary circuit. This is the reason you have trouble adjusting the mixture screws and have fuel dripping from the boosters.
You'll need to drill the primary butterflies with a .125 drill bit. If that doesn't get them closed enough then drill the secondaries. If it's still not enough drill the primaries larger, to about .135 and repeat until you have no more than
.040 throttle opening at idle speed. Adjust your mixture screws each time. It's also possible you'll need to decrease the size of the idle air bleeds. This will fatten up the idle circuit to be compatible with the increased air flow. Measure your idle air bleeds before you start (these are the small brass jets at the outer edges of the air horn) most likely they are .120 or .080. Go down to about .070 if they are .120 or .040 if they are already .080. Do this only if you can't get the mixture fat enough. And do this only on the primaries first since your carb doesn't have mixture screws for the secondaries. If you have to do the secondaries don't go too small, but you can always open them back up.
Run a 160 stat, the 195's were designed to keep the cylinder head combustion chamber temps up for emissions in newer vehicles. You don't want that, you want them cooler. You may also want to consider an auxillary electric fan in front of the radiator for idling in heavy traffic in hot weather.
These suggestions should cure your problem but ultimately you may need to mix a little race gas with the 91 octane to keep it totally safe and out of detonation under any circumstance.
Let us know.
