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If you want to suggest swapping out to Merlins, this is not the thread for you.
Looking at the below image, it seems ideal to:
1) Move the radiators from the wing edge to make space for more fuel.
2) Move the magazines from the nose to behind the pilot, making space for fuel in front of pilot, and significantly larger magazine capacity.
Neither of these should seriously screw up the centre of gravity. However, how do we cool those engines without wrecking the aerodynamics?