For dinner table discussions like this I find it helpful to use one of these Moroso Calculators (about $10) to reverse engineer the question.
Assume the 261 is bored .030" over. Now you are working with about 264 ci.
Assume DeuceCoupe is right saying you have 8.25CR with that 848 head. I dunno - if that is correct then we can proceed.
My calculator says your total chamber cc's are at 99 at 8.25:1. To achieve 9.0:1 CR you are going to need a chamber volume of 90 cc. Cleaning up the deck and head with a .003 cut is going to remove negligible cc's (I don't know the bore/stroke of this engine - if you do then you can calculate this amount more precisely.). I am going to guess it is about 1 cc, putting your final CR at 8.35:1. Today's fuels are of higher octane than when that engine was designed so I'm thinking that ol' bird will do just fine on regular grade pump gas.
