From the 2009 rulebook:
"XO class consists of inline overhead valve (OHV), flathead inline and flathead V8 (except Ford & Mercury) and V-12 engines, 1959 or earlier design, up to 325 cid. Examples include Chevrolet, GMC, Hudson, Packard, Buick, Lincoln and Cadillac. Foreign engines are NOT included."
Color highlight mine,
DW