The "Hemi" six was developed to replace the 251 flathead six that was used in U.S. built trucks thru the mid '60s. There was also a 265 version in the '50s. The original Hemi head mockups for the 1951 V8 was done on the 251 six blocks.
Chrysler had two series of inline sixes until it was decided that the small V8 was adequate for the truck market. Until '55 Chrysler had three different inline six series, up to 413 inches for heavy duty trucks.
The overall length of the "Hemi" six is too long for some U.S. applications. A slant six is 26" long from bellhousing flange to water pump flange.
