I'm also for detonation. I have seen detonation worse than that a few times and it can really hammer the lower rod bearing. Check that bearing for sure.
It sounds like your mainjet is plenty big for the carb size and a cub with those pipes unless you have a very odd needle, you might tell us what needle it has and pilot jet size should be in the 48-55 range.
Check your squish clearance before you put it back together, or even check it with those domes and see what clearance you have. If your cyl was supposed to use for example a .022" (0.55mm) base gasket with those domes and you used a .012"(0.27mm gasket that can change your squish chearance from a safe clearance of say .040-.044" (1-1.25mm) to maybe .030" (0.75mm) which can cause detonation VERY easily. Make sure you use the correct base gasket, piston , and dome.
Andy (M/M) is very good with cubs and can get you the correct domes, pistons, and gasket thickness to make that work correctly. Hope this helps, Radar