Hi Romain,
Selecting the proper camshaft is extremely important - all your other choices are affected (carburetor, manifold, etc.) by your cam selection. If you're going to change cams, and given that you want good gas mileage and torque, I'd go with a cam and lifter kit from (for example) Edelbrock's Performer-Plus series of camshafts.
http://www.edelbrock.com/automotive_new/mc/camshafts/cams_chevy_sb.shtml
Perhaps others can chime in here, but when selecting a cam, it's best to talk to an expert (i.e. call the manufacturer for advice).
Holley or Edelbrock? Personally, I like the looks of Holley carbs and am used to working on them. Either way, don't go too big. I've used both - either one should work fine.