318 X block

In 1979, Mopar Performance started selling their “X block,” a heavy duty racing small block which would displace 340 cubic inches used as is, or could be bored out, as Larry Shepard suggested, to 355 cid (Shepard also warned against using a used Kasper prototype block). These were painted orange and a large “X” was cast into the end. It had standard and four bolt versions. The same block was used in Pro Stock during 1979 and in NASCAR racing , as well as ARCA oval track racing in the 1990-91 seasons. There were numerous features in this block to add strength.

The engines were also used when Chrysler sponsored the IROC (International Race of Champions) series, which uses identical cars; drivers ran with Dodge Daytonas converted to rear wheel drive, with the 355 under the hood.