Gimme the suffix code off the front passenger side of the block (behind the alternator) and I might be able to give you an educated guess.
There is also supposed to be an ID mark on the BACK of the block, but good luck finding it with the engine installed. So far, for a guy I know who was the one that told me about this theory, every time this mark is present, the block is 4 bolt.
I should get a picture of it. I'm not really concerned about being able to ID all of the 4 bolt main blocks, I just thought it would be neat if there is another way to positively ID a block as 2 or 4 bolt, without pulling the pan.
By the way, if anyone has engine blocks laying around that hasn't already given me the suffix codes, I'm collecting suffix code/casting number/2 or 4 bolt information for SBC chev's, to see if the suffix code ID's a 2 or 4 bolt main. Not enough samples yet to make a call as to whether it's reliable or not.