[kennyd]

According to my manual, is the following. If I subtract the Steering from the main it does come out to 4.1. Does this mean I can use the smaller valve?

I think your implement flow is 4.1GPM, the the 5GPM Cessna Compact valve is the one to use.

JIC fittings are the best to use over NPT, they are easier to install and will be less prone to leaking.

Two very appealing qualities to me!!

Take a look HERE for a good description of fittings. You can also look around their site to see what the different fittings look like.

This would be the existing PB feeding the BH ? Correct?

This would be from the new PB sleeve on the new valve? Correct?

This is where I get confused as I have not seen one of these valves installed.

My impression was the PB, Return and sump lines from the tractor were all somehow connected to the valve.

OK, if you take the BH off, you have a PB "LOOP" that must be connected back together correct? You can just plumb the new valve into this "LOOP", you do not need the PB sleeve for the valve.
You can take the PB line from your FEL valve to feed the new valve, then you will have to find a return port on your tranny to dump the fluid back to the tank.

Keep in mind that I am not familiar with your machine, just hydraulics in general-so you will have to decide exactly how to do this. Can you get a shop manual?

You can read the many threads that MadReferee posted about adding valves to Kubota tractors, that should also help you understand the theory.

Even with the extra 50 for Cessna valve, that is 150 more than I come up with. What am I missing in my calculation?

There will be fittings, and hoses that will add up to $150 very quickly. There is nothing about hydraulics that inexpensive!