I have a friend with a 66 Mustang with a 289. He knows what the firing order is, but how do you know what order the plug wires go on the distributor?

 

#1 should be marked on the distributor cap, and the rest just follow sequentially around the cap in counter-clockwise rotation.

 

If the #1 is not visible, how would it go?

 

Take the plug out of the #1 cylinder (front-passenger side). Turn the crank clockwise with your thumb or finger covering the spark plug hole. You'll feel the pressure as the piston comes up on the compression stroke. Stop the crank when top dead center in indicated on the harmonic balancer. Pull the distributer cap and see which post the rotor is pointing to. Use that one for the #1 plug and put the rest in firing order counter-clockwise as Conanski mentioned. The cylinders are numbered 1-4 front to back on the passenger side and 5-8 front to back on the driver side.

 

Thanks for the replies!! My friend went out and got a new distributor and got his Mustang runing last night.