Possible Mystery

[cv70chall]

We're still dealing with the heat issues and working to get the exhaust manifolds to run cooler, but in the meantime, we noticed that the spark plugs are actually 5/8" and not 13/16" as one would expect.
Researching this told me that 5/8" plugs are usually found on 400 Heads and NOT 440 Heads, which concerns me.
Should this be an issue with the engine or are 400 and 440 heads generally interchangeable?

Thanks-

[gyman98]

The size of the spark plugs tells you nothing.  You need the head casting number to know what you have

[OooohBarracuda]

If there is room, it really doesnt matter what hex-size plugs you use, provided that the reach, thread pitch, and heat range are correct.

[anlauto]

Quality NGK plugs have a 5/8" head.

[cudabob496]

hot exhaust, too much advance

[cv70chall]

Here's the numbers I was able to locate.   #1468 Any ideas?
I actually advanced the timing a bit- and the manifold temperature dropped by about 110 degrees.