273 Commando Engine Build

All the OP wants is 300-350 hp which is about 1.08-1.26 hp per cid, fairly realistic numbers.

Hp is basically airflow over time aka cfm, which heads and cam is mainly responsible for. Cam wise you'd probably want to be on the tamer side while accomplishing your goals of 300-350 hp, somewhere in the 210 to 225 intake duration. So you want the heads to do the majority of the work. Stock heads with that cam range and decent cr probably get ya 250-300 hp, mild head work add 25 hp, fully ported get you there and more. Stock 360/magnum heads and a 225 cam easy 350+ hp, ported either more hp and or smaller cam to reach 350+ hp.

Now displacement, a 273 is gonna make these peak hp numbers in the 5500-6500 depending on, A. how much peak hp and B. how efficiently you make it, tq per cid. Now the trick is to make these higher rpm power without sacrificing low rpm driveability, Now the more gear and stall you run the less this a problem. Optimal gearing for the 1/4 mile for these are probably in the deep 4's so if running around 27" tires with 3.23 you'd probably want to stay around a 210 cam, a 225 cam I'd want at least 3.73 and 3000 stall. Don't forget tire size is a gear ratio, a 24" is 1 to 1, 26" = .92, 27" = .89, 28" = .86, 29" = .83. A set of 3.23 with 26" tires is the same effective ratio as 3.55 with 28.6" tires.