"XO class consists of inline overhead valve (OHV), inline flathead and flathead V8 (except Ford & Mercury) and V-12 passenger car and pickup truck(or the same engine design family, 4.N) engines, 1959 or earlier design, up to 325 original cid. In the spirit of the class, XO engines shall typically be those run at the Southern California Dry Lakes in the 1940s and early 1950s. Examples include Chevrolet, GMC, Hudson, Packard, Buick, Lincoln and Cadillac. Foreign engines are NOT included."
You have to read the complete rule, not just what you want. I fail to understand where, in bold print, it mentions an engine design which competed as new in a '60s car is legal.
DW