Pop pop on exhaust side

Idle timing at 18* is not too high & not causing the problem. Complete rubbish. Plus OP says he has adv & ret timing, problem still there. Millions of cars of that era had 20+* of timing at idle.
I would first check the p/up gap in the dist. Make sure it is about 0.010". If gap is too big, or reluctor touches p/up, no spark on THAT cyl; unburned gas into exh system, gets ignited in pipe, pop.....Remove the dist & check each each cyl for gap...or any looseness of the internal dist parts.
Next check & measure the resistance of the plug leads, with the lead still plugged into the cap. Check the rotor & inside of cap for any signs of arcing.
I would replace the spark plugs with NGKs in a 5 heat range. A plug that is too cold will also cause your problem.
Could also be as others have mentioned: carb related, broken v/spring, valve not seating because of incorrect lifter pre-load.....or wiped lobe.