WTF does RPM have to do with intercooler size?
Let me answer that: Nothing..
It's psi capability, flow-rate, efficiency, and psi drop. RPM has absolutely NOTHING to do with intercooler size, period.
If anything, the Cummins FMIC may be too big for 8-12psi, as the core is of a large volume, and there may be 1-2psi drop in boost from SC to intake manifold.
I'm not trying to be an ass, but people shouldn't post incorrect information.
Something like a Precision AS1025 (~$360) would be plenty efficient, as it's fairly compact (31.50L x 8.00H x 3.50D, 2.75" Inlet/Outlet ) and will support 600hp as an Air/Air FMIC.
I've got no affiliation with Precision Turbo, other than experience with their product, having used it many times in most of my previous builds.
My last build was a 550whp Subaru, using a GT3255RE, PTE AS1025 fmic, 27psi, 880cc injectors, water injection, worked 2.0L heads, 2.5L block, 9.7:1 CR, billet rods, knife-edged crank, B&BP, walbro 255, ported Cosworth oil pump, TGV deletes, gasket-matched porting and much more.. All machine work by Howard and Tommy @ ARFab (now CNC Engine Dynamics.)
That's 551 whp, 607 lb/ft TQ @ 5800rpm