one way to be absolutely sure (as older heads could have been put on a newer block, etc...) I would use the casting number on the engine. Here's a link to where you can look that up.
SBC CASTING #s
Of course if it is leaking as bad as you're saying, and you plan on doing head gaskets, it may not be much more work to remove the motor anyhow. Depending on the equipment/help you have at your disposal.