100mm should be pretty close from that picture, maybe a little less as it's not centered on the top, it should have been on the right side there, but that's impossible without cutting it.
I think one problem is that the engine is going to need to breathe through a slightly small manifold for the size.
So I would absolutely run a conical manifold and try to see if it's possible to remove edges or anything other restrictive.
Neal has experience with these engines, he could probably give you more hands-on experience.
But I can give you my take on it, but in that case I will do so in a PM.