[Excaliber]

93 octane should be fine, it was mentioned all ready the KEY is to determine if it is pinging under deep throttle (detonation, knocking it's known by many names). A compression ratio higher than about 10 to 1 generally requires you to do one of two things:
1. Run race gas or aviation gas which has a higher octane count than premium fuel. Doing that is a real drag, hard to get gas, expensive and generaly a pain in the butt.
2. Retard the ignition timing and NEVER 'lug' the engine, keep the rpms about 2500 at all times.

As to unleaded or leaded, no big deal. If you have the old leaded seats I wouldn't worry about it UNLESS your taking an extended trip at freeway speeds, several hundred miles in a day. Most of us never do that with our Cobras anyway. The extended run time at cruise rpm eventually gets the valve seats hot enough to cause damage, otherwise a bit of stop and go allows them to cool off.

Determing your compression ratio is going to be all but impossible without the engine build specs. Listening for the ping and adjusting timing accordingly is the only logical
solution here.

It's pretty hard to hear the ping over the roar of the sidepipes, the ping should be most notable when starting off on a hill at low rpm. If it sounds really loud with only moderate throttle when starting up the hill, yup, you got high compression. Depending on the hill, throttle and clutch slippage SOME light ping may be expected anyway.

I was running 12.5 to 1 compression, thats REALLY HIGH and was able to use 92 octane gas OK by retarding the timing, avoiding taking off on a hill (steep incline) and never letting the rpm's drop to low. HOWEVER, when I would run race gas and advance the timing the engine made noticably more horse power, ran better, much quicker and more comfortable to drive over all.

NOTE: That engine had forged pistons which are much stronger when it comes to 'ping'. Cheaper pistons will crack like an egg if they sustain much pinging. Also it had iron heads, alloy IS better for this problem.