4spdragtop and his pictures explain it all. Although places may claim that Offenhauser will work on the 340 360 the pictures show reality. The manifold was designed for the 273. Then the 318LA came out and it works fine with the 318 (Uses a variation of the 273 heads.) The 340 comes out in 68 and its ports are SIGNIFICANTLY larger than the 273/318. Thus the problem of insufficient sealing of the Offy on 340/360 heads.​