I have not max ported a 318 head with a 1.88. With a 1.78 valve they kind of stop in the 215-220 cfm range.
I think 230-240cfm with a 1.88 is about it.
I did some 'not all out' 1.88 273 heads and they went around 225 cfm. They end up near 140-145cc and that's tiny for anything but a 273/318. I don't think they would be the best choice for 6500+ rpm and or more cid..but it depends on the parts and pieces I'd suppose.
You need so much volume to be able to 'refill' at that rpm