I think original heads could do with a bit more meat up top anyways, as the top seems to leak first usually. Reason is the location of the spark plug on the top, which 14mm hole make the head more prone to warping between the two top stud mount.
Warping makes the heads leak because of the sealing that is compromised then. On a sidenote: The unequal head stud lengths (tops are shorter, still they leak there first) has obviously no meaning in this. Took me a while to realize that...
You may have noticed on original heads, that the top fin is cast a little thicker there in the middle, probably to reckify just that issue of the weaker top area...
Then we make the heads even weaker by opening them up and/or machining them down for CR reasons..
Another suggestion: look at the '2,6ltr heads' of Klaus on his website where he has made/cast-in extra material and finned that to both create more strength AND retain cooling properties. Imho, thats the direction you always need to think into with designing a better type 4 head.
So, what I am saying is: by all means add more material to the top, whether it is for 5th or 6th studding or for more structural strength. But with the latter you adresses the problem, while extra head studs only needs to be done if you run out of other options imo. Since yu are at the design stadium, I would try to tackle the original problem (head strength related to cooling) first.
In that respect: Adding an extra head fin is a very good idea still, as this also both helps strength AND cooling. I gladly trade in a cast iron fin of the barrel to an extra head-wide aluminium fin on the head.
Also look at the 356 head and how many and how large those fins are, They were only 1600cc/90 horse engines. We run up to 2,7 engines with our heads!
Really, cooling is everything. Surely you know all this already, so please let it come back into your design as well.
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