[bigblockv6]

For the most they are stamped with an M but not always as is the case of the one you have and it could be just the opposite. Ed Snyder bought a 401 with the M stamped yet it had the small port heads, this could be an issue with replacement engines rather than ones that came with the truck from the factory. The heads in pics are definitely Magnum heads judging by the intake ports and the larger size exhaust manifolds so your good. Look around for dates that can be cast on top of the back part of the intake manifold or the exhaust manifolds, if they're original to the engine then that will give you an idea of the year. From what it looks like to me this can be a 1968-71 engine, the breather is not on the left rear of the block rather it is plugged like all 68 up engines.