Higher octane allows you to advance the timing. Just go a couple degrees at a time, and be careful! It's hard to hear detonation in a Cobra, as they are usually pretty loud. Keep an eye on your total timing as well, usually having everything in around 3000 rpm.
Usually, just draining the radiator leaves quite a bit of coolant behind. If you really want to remove all the old coolant, try flushing or vacuuming, via a coolant machine. Also, when refilling, if possible, vent something high up in the coolant path to purge air as the new coolant fills up. Watch the level for a couple heat/cool cycles, as it will usually require topping off.
Now, go drive!
~Dave