the number you want is 'Cast' not stamped, can be found below or near the fuel pump. Also get the 'cast' number off the head will help. Could be a 261, look just above the starter for the 'II' captains bars, but some early 235s had this marking.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed