As we all know the Chrysler hemi, 1st generation, came out in 1951. the next development of the hemi-based engines occurred in 1955. a smaller 301 cid version of the Chrysler hemi engine was released. It had a bore & stroke of 3.625 X 3.625. It also didn't have hemi heads. It had what Chrysler called the Spitfire poly-spherical cylinder heads. The combustion chamber wasn't a true hemi, but the valves were canted, and they were still located in a cross-flow arrangement. Rather than being directly opposite as in the hemi, the valves were diagonally opposed in the poly head. In many ways this cylinder head design resembled the well-known big block Chevrolet engine introduced in 1965. These heads were also less expensive to manufacture than the full hemi heads and they also used one rocker shaft per head. Interestingly, Chrysler's 1999 Jeep V8 engine has a similar combustion chamber design. The 301 cid Spitfire was replaced in 1956 with two 331 cid versions. The two barrel version was rated at 225 hp, and the four barrel version was 250 hp. A 1957, 354 cid engine became the Spitfire, while the regular hemi was bored and stroked for 392 cid. In 1958 the 354 cid poly-head engines were also up in horsepower, to 290 hp & 310 hp, but it was their last year. The poly head engines, Spitfire, can be identified with their "wavy bottom" valve cover. There were other poly head engines of different sizes but only the Chrysler ones were called Spitfire. The Spitfire and hemi cylinder blocks are basically interchangeable from similar displacements. So basically, if you can find a set of hemi cylinder heads for the size engine that your Spitfire is, just swap them. Check it out first, Hot Heads is a good place to start, to make sure there are no small details that need attended to first. Parts are still available for these engines through a couple of suppliers. There's nothing really special about them but they are not seen that often and might be worth the extra bucks it takes to rebuild it. Or, turn it into a hemi. Hope this helps and good luck.
