With the Edelbrock and any other "metering rod" carburetor, you have to figure in the various diameters of the metering rods, too. The 'economy" (larger, toward the top) and the "power" (smaller, toward the bottom) diameters. Those plus the jet sizes and the "power valve spring" tension makes the calibration for the primary side of the carb.
The "road load" and "unloaded" test at 2500rpm would give you the "economy" side of things, being that the engine manifold vacuum should be above about 17" Hg. "Road Load" would simulate what the engine would be at during a level-road highway cruise event. So no matter whether "road load" or "unloaded", the vacuum level should be high enough for the vacuum advance in the distributor to be maxed-out, with a total advance (vacuum + centrifugal + base timing) at this part-throttle situation near 50 degrees BTDC.
The "loaded" situation would have enough load to put the intake manifold vacuum close to about 5" Hg, which would most probably be WOT. THAT'S when the 38 degrees (or a bit different for an LA motor) figure would be operative (centrifugal + initial base timing). Just to clarify things.
How many miles are on the plugs? They look new, from what I can see from the pictures. A "leaner" mixture would keep the insulators "white", but when the mixture is too lean, it becomes a chalky white, rather than a smooth white.
When plug reading was discussed back in the '60s, the racers had a special viewer/light that they used to look at the base of the insulator, rather than just the exposed tip.
On my various 383s, the 2bbls spec'd Champion J-14Y, the 4bbl spec'd Champion J-10Y. The '72 400 spec'd J-13Ys. In normal daily driving, they'd all have a light tan/white insulator tip. With their stock jetting. The 360 had something like a "9" or "11 heat range Champion, I believe. Not sure how the heat range of your plugs relates to that, but as long as they are spec'd for it, whatever works.
Once, I was trying to better adjust the idle mixture on one of my cars. I'd put an OEM-spec Holley on it with a Holley Z-Line intake, with a slight cam upgrade. I had it hooked up to the BIG Sunn diagnostic roll-around we had in the shop back then. I was trying to get the CO down to what they claimed it should be, BUT when I got close, the idle got a little shakey and the digital HC readout went nuts. I'd hit "lean misfire". Not a real "miss" but not a full "hit" either. As soon as I rickened it back to where it was, the readings settled down to normal. So, "lean misfire" is not the same as a bad plug wire miss, or similar.
My own personal theory of plug heat ranges is to use the equivalent of the OEM heat range in whatever brand you like. BUT if you want to add a bit more advance into the mix, then going one heat range colder might be advisable. Which would keep the insulator tip a bit cooler with better combustion heat dissipation through the spark plug body.
Keep us posted on your progress,
CBODY67
