What piston stop for a big block?

What's the best one for an oem head, 9:1 engine?

I made my own on the first one, I knock all the center porcylen and center electrode out of the spark plug and inserted a long 5/16( I think that was the size) bolt with a nut on the cylinder side to lock and hold the bolt into plug body and used it as my first piston stop with the head on.
You can buy them from most any tool truck, they are usually brass or bronze mix also which is good
one of the ones I've bought had a small hole in the center to relive the pressure when moving the piston up towards TDC, I think it whistle also

Yes get one with the hole in the center.

I see 14mm and 18mm ones listed but no sae?

What motor are you wanting the SAE one for? What size and thread count do you want?
All the American made car I know of use either a 18MM or a 14MM thread spark plug

BB Mopar.
A lot of the ones I saw listed on Amazon said small gas engines and chain saws so wasn't sure if I'm missing something.

14MM

Spark plugs went metric back around 1930.

as I recall spark plugs were invented overseas in England that's why they are metric??

Harley plugs 18mm: 1909

All the flathead Ford motors I've seen including the 1923 Model T 4 cylinders had 18 MM plugs, maybe some of the earlier cars ones had bigger ones

Not sure where they were invented but Albert Champion was a French guy who had a lot of early commercial success making plugs. I think he went on to be the founder of AC Delco also. Some early Fords use pipe threads for plugs but gradually all the popular stuff moved to metric.

Of course....... I don’t think England was on board with the metric system “back in the day”.

You are correct on that Them dang English and their English standards
That is not to say we Americans have never done that type of thing to go against the worlds standards

Hey ...we still aren't
Actually, we're kind of schizophrenic - still universally use miles and mph, milk sold in pints (let's not go there...), sometimes use lbs weight....but at school even oldies like me did all my maths, and Engineering degrees in metric, and pretty much everything else is metric too. I'm just about used to deg C in the weather forecast....but I speak deg F too