350sbc pops and spits under load

[JointTech]

just picked up old blue from the muffler shop. (headers and cherrybombs)
It popped and spit and generally ran like crap on the way home.
In neutral I can rev it no problem. in drive it will randomly die and fire and pop. Doesnt seem to matter if its heavy load or light/ fast or slow.
I just bought this thing and havent even changed the oil in it yet.
It has HEI ignition.
I did just get cap/rotor/wires/hei unit from Summit which Ill install with new plugs and an oil change this weekend. I havent done any timing in 15 years but I think I remember how.
I'm thinking timing or maybe the carb? I was way low on gas and only put in a couple gallons after leaving the shop. I wonder if I got some tank crud in the carb since it sat awhile before i bought it. I have driven it 20 miles or so since buying and it didnt do this until now.
I was also thinking plug wires not in order since they pulled them to put on the headers. But it purrs like a kitten when its not popping and spitting.

thoughts?