not sure where exactly your cooler is located, but anything like that wants to be very close to the radiator, and not away from it - this promotes through flow, whereas having it too far away inhibits flow by creating a turbulence zone - you don't want it touching, as that could induce conduction, but 1/2" is about right
as for size of radiator, there is a formula for that, but it's been at least 25 years since i used it - the V8 will need more volume than the 3.0, but it won't be much, and more than anything, it will be more rows needed to increase the surface area
the shroud will be the key - it needs to fully encapsulate the rear of the radiator, and then having a fan that can move the appropriate cfm, and not get in the way of the flow (i.e. terminal velocity of a propeller) - while not likely to be a problem in any 968, high speed cars need very high cfm fans, or they can actually create a high pressure zone behind the radiator - i've seen systems that actually cut off the fans at certain speeds, and others that had a relief flap that opened up at certain speeds or was controlled in the cockpit
also cutting off extraneous entry of air into the engine bay, and forcing all air to pass through the radiator first, will provide the best negative pressure zone and the best flow and therefore cooling - the 3 factory belly pans to a pretty good job of this - the V8 won't let you use those, but you can create something similar
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating
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