The 226 should have OHA or 7HA cast on top of the head. The 254 will have 8HA cast on the head. On the block between the generator, and the oil filler tube, just above the pan rail, is the block number. The first of the engine serial number will be 7HA if it's a 226, or 8HA if it is a 254. If it's a 254 grab it, there is a rather small almost cult like following for this Flathead 6. Some say they were Henry's best kept secret, and the way my 226 has continued to suprise me I have no reason to doubt that statement.
Good Luck