Every straight 6 I know of is 1-5-3-6-2-4
As mentioned, pop the cap and watch the rotor as you bring it up to #1 TDC, then wire it. Make sure the cap doesn't have any carbon tracks too.
Other things that causes no change when a cylinder are intake leaks (very common), valvetrain issues (not as likely, due to the compression), and ignition problems.