Normal firing order for most six cylinder vehicles would be 1-5-3-6-2-4, (although some could be 1-4-2-6-3-5). I would suggest that you try 1-5-3-6-2-4. You need to set the engine for No1 cylinder on its power stroke (ie induction, compression, power, exhaust). This will then show you via the rotor arm where the number 1 plug lead should go. Then, according to the rotation of the distributor (ie clockwise or anti-clockwise) you should fit the remaining plug leads.