500 miles later with my DIY 318 rebuild...

An O2 sensor is the ONLY way to get an even close approximation of what the actual fuel/air ratio is. Reading plugs used to work OK with leaded gas and assumed the heat range was cool enough. Modern fuel burns a lot cleaner and OEMs notoriously spec a hotter than necessary plug for daily driver use. This makes sure the plug is always clean in stop and co running and frequent cold starts. Great to prevent misfire related emissions. Without a wideband I have had the best luck by first returning the carb to the factory jetting and configuration, paying especially close attention to float levels. Then dialing in my timing. Then and only then do I start messing with the PRIMARY jets (with secondaries disabled if possible). On aftermarket carbs, I start by going leaner 2 numbers at a time until is surges, bogs or becomes noticeably slower. With factory carbs I generally start by going richer. At my 2800 foot altitude aftermarket Holleys are a bit rich out of the box, Edelbrocks are close and OEM Quadrajets can be anywhere.