235 heads do not have the small steam holes that a 261 head has.Easy to do with a drill however.Later 261 head casting numbers end in 850.There are web sites to check the casting numbers on the right side of the block,try www.inliners.org .A 261 has beefier rods with larger piston pins and a bigger bore than a 235.The rest is pretty much the same.Some early 235 engines had the bars down by the starter.