I have a set of Dart 320 heads and put a flat gasket plug on were it takes a tapered plug.not sure why but I did put a tapered index washer on the flat gasket plug so it did not fully seat in the head. But after running the car it started to misfire so we took the plugs out and noticed that the crown of a KGN plug was tapped and the crown was busted off, is this something I should be worried about or just run the motor as is...

Put a camera in that hole and take a look at everything. If the piston and cylinder wall look ok then make sure you didn't bend a valve. If ok put the correct spark plugs in it and try it. If there is damage to the edges of the piston I would tear it down.
Have you inspected the other 7 spark plugs,are they ok?

leakdown and copression then go far it

Older Dart 320's take a SHORT reach tapered seat plug...

I learned this the hard way when I bought an engine, a 509 with those heads and when the seller offered to install some plugs for the ride home....I said no don't worry about it...!!!

Dart website shows long reach gasketed plugs.....but they changed that at some point over the years

I made some runs with long reach gasketed plugs and banged up the plugs at higher rpm....pretty bad...

I pulled the heads.....ground a flame notch in the pistons and fixed the spark plug holes and used the right plugs....there was no other problems with that engine and it ran for years and was sold running

If I recall right I pulled a piston or 2 and found no other issues....

The only complete engine I ever bought and it was because I was very busy doing other peoples engines and had just bought another car and needed an engine for it fairly quickly....

I put index washers on all the gasketed plugs and had the same results it broke at the tip of the crown on all of the plugs right above the threads,threads were not hurt.
Trying to send a pic of the plug. can I get your e-mail????

Look closely at the spark plug hole and you can always tell if the hole is made for a flat gasketed plug or only a tapered plug by looking at the seat area...

The old Dart 320's took ONLY short reach tapered plugs.....an unusual plug for a BB head.
Some old iron BB heads also used them

Peanut plugs is what we called them...

No reason the right plug should hit anything....even if it's not indexed unless the heads were milled a ton or the piston domes are huge with no flame notch....

PM sent with my e-mail