Yes, you could put another 4 speed behind the one you have but why? The linkage alone would be a challenge. You might find a 5 speed overdrive but likely the ones made for your truck will be direct in 5th. If your looking for more speed think about going to a 2 speed rear, assuming you don't already have one. Most common rears were 6:50/7:17 ratio. Twin trannies, AKA brownies, were not used in such a light duty truck.
