72 360 first start question

Lean conditions, ignition timing and valves not closing or at the right times/or at all can cause backfires out the carb.
Advanced or retarded either one can also.

I'd verify the firing order and ignition timing first, then on to fuel and valve timing.
It's also very easy to accidentally swap the cylinder number 5 with 7 wires, and that will also cause poping.

Short fires are not a big deal usually if put out soon, and the car won't explode.
If left on fire it will boil the carb over and make a bigger fire though.

Keep a wet towel handy if it makes you feel better, and that way you can throw it over the carb if the fires don't go out by themselves within a few seconds.