I would think that you may have overheating issues on a hot day. Since the Spitfire has zero cold air intake, the crankcase, lower cylinder, exhaust, and carb will all run in a very hot environment. The radiated heat from the muffler will make the the bottom end of the engine run hotter and will probably vapor lock on you. The Mustangs have a some flow through with the chin scoop, and in flight there is a blast of cool air on the entire cylinder of the engine. Also the flowing air has a much better chance of sucking some of the hot ambient air inside the engine compartment out as it enters and exits the cowl area.
The Spit....no flow, no way to extract hot air.
My 86" Jeff Foley designed Me109 is setup very similar to your Spit. The original had DA-50 and had to have a Naca-ish shaped type cut-out directly in front of the cylinder. Even then, the mixture has to be perfect and it still gets HOT!!! So hot, that that glass fuse above the engine gets hot to the touch and the carbon fiber spinner gets hot after a flight. I am using a G-62 in mine since they are more tolerant to heat with their larger, thinner cooling fins. I am also trying to run a remote muffler mounted behind the firewall, in the oil cooler area in an effort to remove a major heat source away from the engine.