[Kurt S]

I think I've seen 2 other L48's, out of hundreds, with 186 heads.
They are original to the car?

The small block cooling system is well designed. With that rad, I'd go with the stock impeller. More flow is not a good thing - cavitation and other problems. The thermostat is still the restriction in the system. Removing the thermostat increases the flow, but that doesn't turn out well.

btw, making hp doesn't mean you'll have more heat rejection. If the engine is efficient, you could have less.