I believe any head other than the original 273 closed chamber head will lower the compression ratio, which will thwart your HP goal. BTW, 300hp is probably not realistic —275hp would be a very extreme build for the street. More things to consider: You need to factor in the weight of that '49 compared to a narrow-body A. I would imagine as a body-on-frame it is considerably heavier. A high HP 273 build will have a very high RPM torque peak (factory Commando was 4000rpm!) — it will not have the kind of low-end grunt that moves a heavy car off the line. I have a factory Commando 273 in a 67 fastback (about 3400lbs) with a 904 and a 3.23 SG — it will not spin the 215/70 tires. You will need a super high stall convertor and a 4.11 rear to get any kind of launch.
