The 340 heads I've used are all pre 1972 or '73 I'd say. It's been awhile since I put one of those little motors together for hot rodding. The 273's will scream if built with good heads, big cam, pistons and good carburetion. My first one in my little '63 Valiant ran 12.60 at Riverside with a Holley three barrel and wheel well headers and a 4 speed, gears, old cheater slicks and was a daily driver. Couldn't tell you what the casting numbers were though, it was a long time ago.