Deve,
As Jerry posted, the 261 head resulted in lower compression than found on 235s with the "848" head from 1956-1963. GM/Chevrolet did increase the 261 compression a little between 1954 and 1963, but the 848 head 235s always had higher compression.
If you want
the various compression specs for 235s and 261s from 54 (or 56) to 63, let me know. I'll try to look this up next week ("spring"/winter break for me).
