choosing the bore: more capacity = more power. also, it gives you scope for future rebuilds, like getting the 86.5 now, if you ned a rebuild later, you can bore it out to 87, not that this is a realistic thing IMO
cooling channel, i have no idea, but im guessing its size depends on what piston bore you get, to maintain a certain thickness between it and the nearest cylender. might be a restrictor for the cooling system somewhere?
might i ask how much this is costing you roughly, and why you didnt just get a local RB30 built up?