The firing order is right, 1-5-3-6-2-4, but be sure the #1 wire is positioned over the rotor when the #1 cylinder is on the top of the compression stroke. (Both valves closed, piston at the top of its stroke. It doesn't matter which cap tower gets the #1 plug wire, but the rotor MUST be pointed at the right wire when the cylinder is ready to fire.
Here's an idiot-proof method of finding the right spot to start the sequence :
Remove the valve cover
Watch the valve springs on the #6 cylinder (NOT #1)
Turn the engine clockwise from the front, using the fan with the spark plugs removed, or roll the truck forward with the transmission in high gear.
Watch the #6 exhaust valve spring (last one in line back by the firewall. When it compresses fully, and then relaxes, start watching the intake valve (next one in line- - -second from the firewall). When you see it BARELY start to move- - - -STOP!
Take the distributor cap off- - - -whichever cap tower the rotor is pointing toward is #1- - - -start the firing order there and follow clockwise- - - -5-3-6-2-4.
This method works EVERY TIME!
Jerry