Try adjusting the striker plate outward. I was having the same problem adjusting my '52 GMC - it would only latch in the "safety catch" position until I moved the striker towards the outside just a small amount. I know it almost seems counter to what you would think, but the downward angle of the top portion of the striker plate has to cam the rotating portion of the latch to the fully locked position and it cannot do it if it is moved too far inward. You may also have to mover the striker down slightly to accomplish the same thing.
