Unchained,
Thats is as big as they can go if you try porting them u get close to the water jacket. The picture you have is of a Race Aero manifold. The gentry one is alittle larger since it has the water passages to the exhaust housing pass through it. Carson Brummet has all of the Gentry castings and I've seen his motors make as much as 1600hp. Hope this helps.
Steve