Spark Plug Recommendations

The heat range has to be done according to how your plugs look now, you have to read them you have to look at the insulator the ceramic white part that surrounds the electrode, look for a ring if the ring or deposit is low on the insulator then the heat range is ok, if it's high and close the end of the electrode it needs to be hotter to keep the plug clean, this is so very confusing there are so many things to look at discoloration on the ceramic, deposits or spots, but the ring on the ceramic depending it's position is how you know if it's in need of hotter or colder, however it could also be a carburator lean or rich or ignition to advanced, this gets involved.

However spark plugs that do there job and do it correctly and best suited for the job are:
AC Rapid fires these plugs are designed with how electricity works and in it's best fashion, electricity loves jumping from and to sharp points, this is the only plug designed for that.
Iridium spark plugs NGK and Denso make these and are very good, also have a good design for electricity and spark, small thin pointy electrode jumping to a grooved ground with several sharp edges. The only problem with these plugs is they don't last long, but i could care less as long as they do there job great.
Bosch + 4 A good theory, the only other plug i like but the electrode is alittle hidden and im not to thrilled with the theory of the grounds making a sun like spark with a factory ign, and kinda covering the plug making it hard to see.

When a spark plug loses it's sharp edeges, like a champion for example straight cut electrode round that sharp edge along it's circle when it rounds off, the plug is junk as is any plug when they lose there edge, the rounder things become the more energy to light it and the more chance for it to fire a second off and jump real weak, i would upgrade the ignition and gap at 60, you increased the motors ability to make power now light it off good

I'm running an MSD 6A with Blaster SS coil, Taylor SpiroPro wires and Accel cap and rotor. I replaced the split fires today with Bosch +4's. Other than the electrodes being paper thin, the split fires were still in great shape. I have my doubts about the +4's but I'll see how they work after a few thousand miles.

A plugs heat range has to do with how well it dissipates heat. If it's made to dissipate heat more slowly, it's said to be a hotter plug. Like has been said, it depends greatly on the overall condition of your motor. Higher mile motors usually require a step or two hotter plugs due to ring and bore wear which allows a little more oil into the combustion chamber. Oil fouls a plug in a hurry. A hotter plug tends to burn the oil away. A bright white, or glazed ceramic after a thousand miles or so means the plug is too hot. A plug that fouls easily and looks black, sooty, or oily is normally too cold. That holds true only if the motor is in good running condition. A light tan color without any fouling, is just right

As far as plug gaps. I like to set mine at .050. I'm not a big fan of pulling the plugs every month or so to set the gaps. Setting them at .050 allows the plugs to wear a little and still stay within the gap range. The larger gaps will require more regular maintenance to keep them right.