I am in the process of cnc machining out the case to accept the cylinders large transfer and boost port base, I am using one of my race cases, I have these cnc machined out for the full out lake race or pro stock engines. In the pic you can see the differance between stock on the right and the race case, the race case already expands the volume of the stock crankcase but the race case will STILL need a good 5/16-1/4" of material removed yet to meet up with the "Big Momma" cylinders. The rear boost port will easily get 3/8" material removed yet, this is a power maker, the rear boost port in a yamaha cylinder just cant be opened up enuff to even come close to the rear transfer port area and this is where its easy to pick up some horsepower.
I will show next a pic of the race case marked out for further maching to match the cylinder bases, I first have to devcon( a type of epoxy) the holes in between the cylinders.
Just roughed in dimensions as of right now looks like the 68mm stroke is the way to go, best numbers, ease of cylinder port spacing, but further calculating is required yet. Definitely going 83mm Bore!!!