'53, The block casting number confirms it's a 261. That casting was first used in '58, and continued throughout the following production years, into early '63. That head is also the factory casting used on 261s.
From Tim's info, you'll see that the block date code is cast just behind, and slightly above the starter. You should see a letter-number sequence that will start with something like CON4 - ignore that, it's just the conveyor used in the manufacturing plant - followed by a letter-number sequence that decodes to the casting date. To see the date code on the head casting, the valve cover needs to come off. It will be driver's side, just a little aft of the carb.
In any event, great engine. If you are rebuilding, check thoroughly/magnaflux for cracks in both head and block castings. Trucks used hard were somewhat prone to developing thermal-stress cracks.
Doug
