Looking at the distributor from the front, first post to the left of the points access window. Firing order from there would be clockwise, 1, 8, 4, 3, 6, 5, 7, 2.
You can find out in the repair manual for you vehicle.
get the compression stroke up on the number 1 spark plug and look at the rotor inside the distributor cap to see if it points to the number 1 wire on the cap
the one inline with the coil in front to the left
where is number one on a vortex 4.3 v-6 distributor cap
in number 1
Look close, the original cap or a good replacement cap is marked as to which wire goes where. The number one on that flat type cap is the middle one on the drivers side.
number one wire will point to the number one cylinder
No-1 wire goes on the distribitor cap in the 7: o clock position clock wise when standing in front of your vehicle. 18436572
The marked # 1 position on the distributor cap faces to the FRONT
The number #1 plug is the forward-most plug on the left (driver's) side. Trace that wire back to the cap and you've got your answer.
http://www.classiccarauto.com/impala/how_to/images/distributor.png
The wire sequence on the distributor cap just goes 1 thru 6, clockwise, starting from the number one position (which should be marked either on the cap or distributor).