Nothing wrong with a Rochester carb. If it is "professionally rebuilt" correctly, should be nothing wrong with the carb. I did not mention the carb because you said rebuilt so I was thinking spark/timing but it does seem like it could be a carb issue.
What exactly do you mean by misfire?
1. Running rough?
2. Backfire at carb?
3. Backfire at tail pipe?
4. missing? as in; one cylinder not firing? Did you do a spark check on each plug?
5. ALSO as Martin said. VERY GOOD point. There is a hole in the B1 carb mounting flange that must not be covered by the base gasket. Some gaskets have the slot, some do not. There must be a passage slot cut into the gasket from the hole into throat, for vacuum. CHECK THAT ASAP!
If carb has no hole in bottom, disregard.
