I have flow number that show the 273 heads will outflow the 302. As far as hardened seats, unless you are towing and running that 273 hard and lean you don't need hardened seats. Pulled the heads off my 273 with 75,000 miles and no valve recession. 67 Dart, 273/Carter 625, electronic ignition, duel exhaust with Flowmasters, Comp 268 cam with 904 and 3:23's drove from NW to LA and back with an average of 22 MPG at average 64 MPH.