340 Not firing on all cylinders

Do you have a timing light?
Since you don't know where the timing is at the moment do the following:

- make a mental note (or put a mark on the distributor base, just below the cap) of the location where the number 1 spark plug is plugged into the distributor cap.
- Remove the distributor cap and rotate the engine by hand until the rotor points to the number 1 spark plug location that you just marked (the mark on the balancer should now line up with the 0)
- you are now at 0 degrees initial timing
- install the distributor cap and tighten the distributor down (enough so it doesn't turn freely)
- start the car
- use your timing light to reset the initial timing to somewhere around 10 degrees initial this should get you in the ballpark.