my first shevy,head questons

OK this is what I got 84 GMC pickup 305 2wheel drive 4spd auto,(tuck was a 350).First off how can I tell one engine from another(305,350,327),the cubic inches shuold be on the block,, right?, can 350 heads be used on a 305,and will it change the compression(because of chamber size),I ask because I can get a set of 350 heads for free and I can redo them and then swap them on in a day.I use this truck daily so I cant have any down time.I'm not a Chevy guy so go easy on me,this was one of those I can't pass up deals and I needed a pickup,thanks ,,,speedfreak340.:smokin: