I agree with the 'minimalist stereo' approach when it comes to early 911S. Hi fidelity equipment is pretty much wasted on a car like that because it is so noisy. The only thing you'll get is a system that is loud enough to hear well over the engine & exhaust, but you won't hear any of the 'high fidelity' unless you listen to the stereo with the engine off...and who could show that much restraint!
Brad