Probably a 216,I think my 235 I had in a '56 Chevy truck had the valve cover bolts on the sides --it could be a GMC "Jimmy" 270 or 302 six too,I have only seen a few though and dont recall exactly what they look like...I think the motor I.D. codes will be stamped on a flat surface next to where the distributor goes on the block,you should be able to find out exactly what it is by the code..