As far as I know the radiators were the same on the prototype and production Spits. You might be getting confused with the Speed Spitfire, which is also available as a conversion from Aeroclub and has an enlarged radiator.
If the Aeroclub conversion is still available its probably the best way to go as you also get the exhausts, which are difficult to make from scratch, though I don't think the canopy has a flat screen, as is required for the Xtradecal version.
The Pegasus kit is good, although when I've came across them on eBay they have gone for silly prices (but then you need to consider the cost of aftermarket items/donor kits to add to that of the Airfix kit).
I have the Paragon conversion, which is excellent, though expensive, as it has a correctly shaped rudder and hinged undercarriage doors as well as the other parts, which enables any version of the prototype to be built. I'll be using it with the Airfix kit when I get around to it. Oddly, though, it is designed for use with the old Hasegawa kit, which is fine except that the latter, as well as being OOP, lacks the characteristic gull wing effect!
Lastly, there is a superb resin kit out there (I forget the manaufacturer) but it costs around £21!
John