I believe the 850 head is the 261 equivalent of the 848 high compression 235 head. I think with the 848/850 you got 8.25:1 rather than the 7:1 from the other stock heads.

Here's the 1955 article on how to soup up the 261, with tips on milling the head, decking the block, porting and polishing the head, pre-ram manifold mulitple carbs, pre-hei igntion tips, and dyno tests...check it out!

http://www.selectric.org/55chevy/soup.html

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi