Matching Numbers???

Your engine block casting number is in fact correct for a '64-'66, 273CI.
There's a casting date on it somewhere also, don't know where on the early LA's it is exactly.
Here's a '68 340 block pic for reference. http://s203.photobucket.com/albums/aa59/340butterduster/Motor 1968/?action=view&current=P1290010.jpg