I have a 67 with a 273 Commando. I had it rebuilt with domed pistons and hardened valve seats — basically stock, but with a hydraulic cam. The best gas we can get here is 91 octane with ethanol. I found I had to dial back the initial timing a few degrees, but that was it. Note that the 10.5:1 figure is an "advertised" ratio — my understanding is that in practice it was somewhat lower.
