The information I have lists the 54-55 235 which had the pre-848 head at a 7.5:1 compression ratio. 56-57 specifications show up at 8.0:1, and for some reason, the 58-62 engines are supposed to have 8.25:1. To the best of my knowledge, the 848 head was the only higher-compression head during the 56-62 year models so I have no idea where the extra 1/4 ratio came from in 1958. Fuzzy math? Sleight of hand? Who knows?
The 54-55 head can be brought down to 848 combustion chamber volume by milling it .090", but some other tweaks have to be made to prevent valve-to piston interference of the intakes, and some sneaky geometry tricks have to be played with the rocker arms as well. Not a problem for an engine machinist who knows what he's doing, but those guys are getting as scarce as a chicken with lips!
Jerry
