Marc there are two types of manifolds available, there is a billet set you can buy (left manifold) fits the head port and carb inlet size very well and also has a sort of swirl machined in it to aid fuel air flow. Or the cast type (right manifold) which doesn't match either very well, the GP is a bit bigger diameter than the manifold and quite a bit more pricey.
Also a pic of the business side of it all.
Good luck
Thomas