302 firing order...and miss fire...

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I have a '68 Mustang with a 302. Its a new engine, roughly 6+ months rebuilt. Just purchased the vehicle. It starts easily but it "pings" as if it is firing the #1 cylinder when the exhaust valve is open.

I checked the timing and its okay. I noticed that the seller had the firing order in this manner: 1-6-3-7-8-2-5-4.....I know that the firing order for the 289/302 is: 1-5-4-2-6-3-7-8

When I changed the firing order to the correct one, the car didn't even start. I changed back to the initial firing order and it "fired" right up. But the "ping" was still there. I pulled the wires one at a time and replaced them after I noticed if there was a change.

Using the 1-6-3-7-8-2-5-4 firing order the #1 plug wire is pulled the car idles fine and stops "pinging". Any suggestions?

If I do have to check the distributor position what is the procedure to check if the distributor is 180 degrees off?