Hi Ron,
I have seen that Stalwart on several occasions. Having worked on Stalwarts since the late Seventies and pretty familiar with them. I have doubts about raising the overall gearing as it raises ratio of bottom gear when you want to do any off road work like climbing river banks. Great if you only drive on the road. Other thing I noticed when it was in the river is its inability to get over sand banks in low water, I assume because it is only driving on centre wheels. I never asked the owner how he engages front and rear wheels, it appeared that they were not driving when in the water.
Not sure about using the pinion splines for engaging coupling, a dog clutch would be more sensible.
regards, Richard