My Mac's, one turns 7400 with the 18 X 8 and the other was worked on by TKG and it turns 7600 with the 18 X 8 Dynathrust. They both have 12.7mm carbs and CH ignition.

Getting back to the subject. Mac's do not seem to respond so well to high rpm mods.

I think the 18 X 6W will do fine.

I have both a 18 X 6W and a 18 X 8W but have not had them on the MAC's yet.