Forget the junkyards for an engine unless you get a yard in a warm climate. In colder climates, if you find one chances are it would be trashed from freezing and thawing. There are a lot out there. My '62 Impala has a 283 but it is a 327 block. It came from the factory that way. They would grab what was on the shelf if the regular part wasn't there and make it work. It is a 283 because of the bore size. You can make a 327 out of a 283 block. You have all kinds of meat to bore so that is something to keep in the back of your head. My numbers on my block designate 327. Someone on this site (at least I think this site) had some 350 blocks and a 327 block they were going to get rid of. Get ahold of a Rod Shop in your area and talk to them. They usually have bulletin boards up with parts for sale or know someone who will have one. Look locally because shipping would be more than what you could buy on for. As for cost...for a non-running block...around $150-200 complete and maybe a little less depending on the condition. Arm yourself with some block numbers or someone will sell you something that it is not. You can check numbers HERE, RIGHT HERE!!! Click on the left side of it or click on the right side of it. You can even click right in the middle!!!!
Good luck and let us know what you find out there.
I was doing a search a little bit ago for something else and ran across this which may or may not help you out. If it doesn't just throw it in the trash can. http://www.mywebsearch.com/jsp/GGmai...ll+block+chevy