Just wondering if your breaker plate ( or vacuum advance is sticking ). i.e you set timing for what is believed to be 16 BTDC, drive it, breaker plate or advance arm changes, timing drops to 10 BTDC. Seems odd that timing would change like that after driving.
My 71 had the opposite thing happening. I would set initial timing to 10 BTDC, drive it, recheck timing ( vac hoses disconnected and plugged ) and it would be 17 BTDC. I would mistakenly reset to 10 which would now be almost TDC so totally wrong.
After checking a few other things, turned out to be a sticking arm in my vacuum advance. so replaced that with a new vacuum advance from NPD with adjustable vacuum and had all kinds of issues such as hard starting, backfiring etc.. Dont know exactly why.
Replaced with NOS correct part for the car, as well as lubing up breaker plate while dizzy was open and everything now works as expected.
Cant comment on carb/engine changes mentioned in the previous 13 pages since I'm not at that knowledge level. Just wanted to pass on my experience.