I would start by seeing what socket size fits the head bolts if you’re really not sure. 7/16" would be 10 hp and below, I believe. 1/2" would start at 15 and go up…..Maroon, if original paint should be 1956 to 1958. I’m sure someone will zap me quickly if this is wrong.

Also, the 18 hp. top two head bolts are 2.0 inches apart, center-to-center.