Much of the heat range consideration is going to based on your use. I recommended the RN12YCs based on the fact that, well, I tend to run engines at high RPM a lot. My first high-performance Mopar was a 340 Challenger with 4.10 gears and a 4-speed and it spent a lot of time near the big end of the tach. It ran better on RN11YCs, one heat range colder than recommended. I also had better results with the J-series projection, rather than the longer N tip for reasons I won't explore.
I guess the best recommendation I can give you is to go with the plug designed for your cylinder heads as far as thread, tip projection, etc. In most cases for FMJ cars, this is going to be the RNxxYC. If your engine and gearing are stock and your driving habits are of the daily-driver style the factory expected most of these cars to see, then go with the original heat range recommended. I would stand by this recommendation even with the addition of a 4-barrel and free-flowing exhaust if you generally just motor it back and forth to work, Church, the grocery, etc. using the extra power primarily for passing and a once-in-a-while blast down on the freeway or empty side street.
If you've made the above mods and tend to run your car hard regularly, then you'll want to explore a colder heat range--318-4V police-fleet cars ran the RN12YC rather than the 14, because they beat the snot out of the patrol cars regularly and the 12 just plain performs better in that situation. If you tossed that 2.27 rearend and replaced it with 3.21s, you're going to be spending more time at higher RPMs period, so you might want to look into the colder plug, particularly if you've done anything in the camshaft department. Combustion is an extremely complex thing and in all honesty, experimentation is the best way to find the right plug if you don't have extremely high-end computer simulators.
The only time you should consider going up in heat range is if you've got a fouling problem. If that's the case, the spark plugs aren't the issue but they will keep you running on all cylinders a little longer until you can address the actual issue, which is usually fuel or cylinder-sealing related.
The only time you should be concerned with the plugs actual design (N vs. J tip, Y vs. J electrode, etc.) is in the case of using aftermarket heads or having changed the pistons for higher (or lower) compression.
In the case of electrode material, I think way too much thought is put into it. I won't get into all the reasons for that statement, but suffice to say your money is much better spent elsewhere in the ignition system. If the man behind the counter tries to sell you a black plastic distributor cap, kindly tell him no thanks. Get one that's tan, blue, or red and has copper-plated or brass terminals, with a rotor to match. Buy better plug wires such as MSD (their StreetFire line is great for the money) or your more-expensive Accels (Super Stock wires are junk). Route your wires carefully; if any two plug wires have to cross, it should be at a 90-degree angle to conserve spark energy (easily done with zip ties). Upgrade your coil to something with some real firepower such as an MSD, Accel, Pertronix, or Mallory. If you're using a factory-style electronic ignition (non ESA), upgrade to an orange unit, or chrome if you run it hard a lot. If you've converted to GM HEI, absolutely save your pennies and get an MSD module for it as OE and parts-store OE-replacement HEI modules are absolute trash, particularly for high-performance use. If you really want to get wacky, index your spark plugs. Then you're really shooting for max power and efficiency, and you probably have a lot of free time on your hands. :icon_winkle:
One last point of consideration: Platinum, double-platinum, and iridium plugs cannot be easily gapped without trashing the electrode(s). You're pretty-much stuck with the gap as supplied.