First time swapping edelbrock aluminum for 915 cast iron heads.

You know, I think there must be quite a few UK-ers out there looking for the truth. I've gotten a few PMs over he past while. I guess it's summer over there now, and guys are planning their winter projects. I hear things are crazy expensive over where you are, even tho you're just a short hop from the west coast of USA. For me the delivered cost of anything from the US is about 50% higher than it would be to a US resident, so that makes things pricey for me as well. But I get free shipping to the border in a lot of cases, and I'm less than an hour from the US shipping depot........by car,lol.

Now I got a question for you
You guys have pure RON gas in grades of 95 and 98 over there.....is that right? Do you know,for a fact,how that equates to USA gas?

I have read that our gas is rated (RON+MON)/2....I get that.
I have read that the MON test is a more stringent test, and lowers the RON rating by 8 to 12 points, some sources quote just 10 points.I get that too.
What I cannot find, is that it's the same gas, just tested differently, and so they just average the numbers, here in America, thus reducing the Anti-Knock Index.
If that is correct, that it's just a test; then 95RON goes to 85MON, and when the two are averaged WE get 90 octane (R+M)/2 also known as AKI... but it's the exact same gas!
Can you speak to that?

The real question of course is : "how much cylinder pressure can a street-guy in NZ safely run, with iron heads ,and/or with alloy heads: and under what circumstances; and with what grade of NZ gas,lol."
And of course, what pressure is likely to cause detonation?, for guys like me that want to explore the limits, and are willing to backpedal,as may be required.
I need to be as sure as is possible.

So far as I've been able to figure out:
your 98RON is (98+88)/2 = 93AKI(US), and
your 95RON is (95+85)/2 = 90AKI(US)
Plus/minus about 1 , in each AKI-grade.