what about the engine casting code on the driver side rear? (raised numbers, should start with a 3) Also look for a code on the passenger side, side of the block, just above the oil pan. Is it a 2 or 4 bolt main? And lastly, if you suspect the cylinder heads are the original or stock off the engine, what's the casting number? (also raised numbers, should start with a 3) Then go check out this site
http://www.roadsters.com/bbc/#blocks