Small bore.
Honestly, I would go with shearers over the CPIs for several reasons.
The shearers pulled better on top end than the CPIs and the routing
of the stingers around the air filters is wonderful. The CPIs just fit
like crap on my bike. The downside tho, the chrome on the shearers
is the worst I have ever seen.
Id get a hot rods, long rod, 4mill stroker. It will drop right in. Anything
bigger and you will have to get your cases machined. And if at all possible,
have a builder re-weld it also. I have heard of numerous instances
of people having them seperate.
Good luck with your build.