Hi Matt - The 'nailhead' refers to the valve size in the heads of both small and big blocks made prior to, and including, 1966 (I'm not sure when they started making them). The block size refers to the 'girth' of the block. Generally, 400 CID and up are big blocks. 350 CID and lower are small blocks. Post in either or both if you want. Most people generally look at all to forums, so you would be covered either way Enjoy!
I'd say its a smallblock nailhead :Brow: . Probably smallblock tech will get you more interest, but as marco said, try both if you'd like.