Another piston option for the 273.

Have you checked out onewildcrazyguy@ how to port 318 (273) heads he got around 205 cfm with 1.75 valves. Which should get you at least 1.7 hp per cfm with street cam and cr (220/9:1) and around 1.15 lbs-ft per cid. The smaller bore might lower the cfm somewhat but 1.7 is conservative.