Need a Mopar guru: 273 part-throttle ping driving me crazy!

When you refreshed it did you notice the pistons? Dome or flat top, 4 valve notches or two? Since it has a Prestolite distributor, did someone put a 2 barrel carb and intake on a Commando motor? You cannot hear the solid lifters in a Commando motor, pull the oil filler and look for adjustable rocker arms. Way out on a limb here, nothing else makes sense.