Do you have the plug wires in the right places?
Turn the engine to #1 TDC compression stroke, then put the wires on the cap in the correct firing order, considering the rotation of the distributor and with #1 wire where the rotor button points.
If you are absolutely sure that is right then start checking each plug wire for spark at the plug while running if possible.
Check to be sure your plugs are good, if it seems to run the same with 3 plug wires off switch those 3 plugs with the other 3, if it still acts the same with the same 3 off then plugs may be OK.
Also check for a bad cap, try another one and see if it makes a difference. Also if your wires look like they have seen better days it would not hurt to change them. If you grab all the wires, or run your hands over them while it is running and have an unpleasant feeling, change the wires.
If compression is good, wires and plugs and wiring order is right then it will run on all 6.
Unless for some reason someone pulled your intake off, stuffed rags in some of the ports and put it back on. Or you have split exhaust and one side is plugged.
What else would cause these symptoms?
