Orignal 225 ??

Your's might be the aluminum block slant six, which I think was installed thru 1963. A good write-up on slantsix.org, and easy to tell since you can see the cylinder bulges on the driver's side of the alum block, or just use a magnet. Problem was many corroded, causing over-heating, and needs a special head gasket which is NA today, other than a special buy on slantsix. For a mostly "correct" cast-iron block, the early ones had 3 staggered "freeze plugs", instead of 5 in a row of later blocks, though some later blocks are stronger.