I'm going to venture and say it's a 360 with an incorrect cast number on the block?
If it's got 360 4.000" pistons, it could likely just be a re-ring magnum, which is common, considering how clean most of them look when they come apart, due to fuel injection.
If it's really a 318 and someone punched it out to a 360 stock bore, you've got a piston that is sitting in a .090" overbore, which is highly unlikely, unless it's been sleeved or something.