If there's a brass float in the carburetor, that backfire might have collapsed it. You might be experiencing a little flooding, resulting in the rough idle. Just a thought, as you've pretty well covered all the ignition-related problems. I believe all the 292's had hydraulic valve lifters, which require a pretty simple adjustment procedure. Each cylinder is brought to top dead center on the compression stroke, (engine stopped) the valve adjustment is loosened until the pushrod is sloppy, and then tightened to just remove the slack. Then tighten an additional 3/4 to 1 turn. (both valves). Turn the engine to TDC on the next cylinder in the firing order, and repeat until all cylinders have been adjusted. Don't try a running adjustment, it's sloppy and inaccurate!
Jerry
