What engine did I just find?

Same thing was asked in the past. Here is an old thread. Basically, they appear to have used the same casting # for std. 318 blocks and their race blocks at that time, which started out with a 318 bore size. But there were added differences where you could tell which it was. That is a std. 318.

http://www.forabodiesonly.com/mopar/showthread.php?t=68184