On the cylinder heads I purchased, the spark plug seat is not tapered. I found this out when I tried to install the tapered spark plugs on the head. The spark plug just threads in then locks up when I try to tighten them. It's definitely not correct. What is the correct spark plug for this head. I believe it has to be the type that uses a washer. Is there a flat faced spark plug that has a 5/8"hex?

This one is the clearest picture I have

The darker area on the inside is not a taper

 

Sounds like pre HE heads - is that what you intended to buy ?

 

@BenKenobi The heads I purchased are exactly like the ones I removed. I do not know whether they are Pre HE or not. Research tells me that some if not all Pre He heads had 3 spark plugs angled towards the front of the engine and the other 3 angled towards the rear of the engine.
I am also aware that some older V12s used the 13/16th spark plug but that would not work as the access hole is smaller and only allows the 5/8ths socket to fit
The heads I purchased have all the spark plugs angled towards the center of the engine. I used some paint on the spark plug seat and installed the spark plugs and the impression it left was about 1/16" when it bottoms out. ( Normally when you tighten a spark plug, there is still some give after it bottoms out. On these heads they just bottom out and you can't turn it anymore).These heads were new out of the original boxes with new valve guides and valve seats. Is it possible that the spark plug seats were not machined to match the tapered seat spark plugs at the manufacturer.
Is a 1/16 seal enough to seal the spark plug? I do not think so
If I have to address this I will have to pull the engine back out and remove the heads and have a machine shop do the spark plug seat or find the correct spark plug with a 5/16ths hex that uses a sealing washer, if there is such a thing.

 

PreHE are the 13/16" with a washer seat seal.

5.3 and 6.0 are the 5/8" with a narrow taper on the spark plug and NO matching taper in the head. Tighten them FINGER TIGHT, and then 1/2 of 1 flat further, NO MORE, job done. Over tighten at YOUR risk, as they will snap off when next removed.

DONT FORGET THE NEVER SEIZE ON THE THREADS.

As we say in OZ, "Gently Bentley and live a tad longer".

The old style sparkers with the washer were 13/16" hex.
Finger tight until contact, then 1/2 (3 flats) rotation to complete the seal.

Worked for me for 40 plus years and waaaay too many Jags.

 

Grant. Thank you. I guess I was worrying over nothing. Your knowledge is priceless.