Pop, miss & pinging-Help

[Slow Build]

A couple of weeks ago I sold my DD so I've been driving my 64. Other than aggravating my back it's been doing fine. Late last week I drove about 20 miles to Walnut Creek. There is a pretty steep grade on Kirker Pass Road and it did just fine going over there. On the way back the truck started making a popping sound like it was backfiring through the carb along with the instantaneous loss of power but it made it over and ran just fine the rest of the way (about ten more miles). The next day I drove to Concord over Willow Pass (not as steep but slightly longer) and on the way it did the same thing. Once I got over the hill it ran fine again the rest of the way there and back. I got in it today and it has a miss. When I try to accelerate at all it starts pinging real bad.

Any ideas on a diagnosis.

It has a two year old HEI distributor that I bought at a Goodguys show. I have no timing marks but top dead center seems to be at the 12 0'clock position. It looks pretty much like where I set it so I don't think the timing slipped but I could be wrong ( didn't disconnect the vacuum when I just checked it.

Where do you think I should I start.