Harry Pellow: Try the Maestro's Patented "Binary Search" procedure to find your miss. This technique will narrow the problem down to the individual cylinder (orside) that's bad in two minutes, making it much easier to find and fix the problem.
Before you do the test, you might want to check that the plug wires are installed properly on the correct plugs, the plugs are clean (or new), and the timing (low and high speed) are in the right ballpark.
First find a flat, straight road with no traffic. Go there.
Go to engine. Disconnect the downcoming linkage rod to one carburetor.
Drive car on one carburetor. It'll drive poorly of course, but a good 356 oughta get up to 50+ mph, given long enough and flat enough. If it does do 50mph, stop.
Connect the linkage arm you disconnected and disconnect the other one. Drive on the other carb. If one side only does 25-30 mph, you have a bad cylinder on that side.
Stop. Get out and pull off ONE plug wire on that side. If you pull off the BAD plug wire, you'll still do 25mph. If you pull off the GOOD plug wire, you won't move! Ergo, you've found the bad cylinder.
If one side does 50mph and the other side won't move, then BOTH cylinders on the bad side are bad (or the plug wire are reversed on that side) or the carb on that side is bad.
Andrew Prill: Let the engine cool down slightly (or start from cold).
Start it up and let it run for about two minutes.
Kneel down and dab a wet finger (spit will do) on each seperate exhaust pipe. The one that doesn't hiss as the water boils is the bad cylinder.
Simple, quick and no running around at 50mph on two cylinders !