Heat an combustion.

Hook your timing light to number 6 and see if the 'clank' coincides with the cylinder firing. Most likely it does, since it goes away with the plug wire removed...

Just thinking, since you had the motor out, assume you had the exhaust off, which rules out exaust gasket leak.

Whatever noise it is has to be related to number 6 firing since it goes away when you pull the plug wire off.

I still say...wrist pin.

Since the noise goes away when the #6 plug is removed, have you checked the distributor cap and rotor? Maybe something funky is going on in there.

muffler bearings aren't synchronizing

Thanks everyone!!!

good one Ben

Got a sound close to that now and I beleive it's y pipe exhaust flappers... At higher rpm it goes away.

I say the bent pushrod was on an exhaust valve and it is now bent again.
If it was a rotating part it would be rythmic - it isn't.
Had that happen before. Time to do a valve job and bronze wall the guides.

I agree that it does sound similar to the sound that counter weighted internal exhaust flappers make. Exhaust flapper noise also goes away around 1200rpms.
That does not explain the prior bent push rod or why the noise stops when the #6 wire is removed.