How to troubleshoot a misfire?

[Xavier]

Picked up a '68 C20 a few weeks ago, have been slowly working on it to make it a solid driver. The motor has been running fine, until last night. I put new adjusters in the headlights last week and did a quick adjustment but haven't driven it at night since. I decided to take the dog and make a trip to a nearby Sonic for some late night shakes and see how well aimed the headlights were. On the trip there the 327 was running fine. After we had finished our drinks I fired the truck up and quickly noticed a miss that wasn't there before. It was definitely down on power, too. This miss was happening at all RPMS, seemed to be consistent in the cycle so I think it's a specific cylinder.

The thing is, this is my first time trying to troubleshoot something without an ECU to give me a clue where to begin. So I'm asking for some advice on how to best approach this or maybe what some common things to look for may be.

The truck in question has a 327 with a quadrajet and the 4-speed manual transmission.