At the moment, the Yanmar unit isn't lighting my fire.
My biggest concern is with where this unit will mount. It looks BIG and space on these tractors is minimal. If you go with a Danfoss unit, you do away with the steering shaft, quadrant gear and drag link. The steering pump mounts directly below the dashboard. Two hoses go forward to the steering cylinder. One hose brings fluid to the pump and another hose provides a way for that fluid to get back to the reservoir. The cylinder gets anchored to the outside of the tractor frame and the rod end pushes/pulls on the kingpin/spindle unit. Very simple, very compact and it frees up a lot of space in an area that is crucial to you. Whether your tractor was originally fitted with a Kohler K or an Onan twin, the weight of those engines does not exceed 120 LBS. You are installing a water cooled 4 banger which will add 350 plus pounds to the tractor. Much of that weight will end up on the front axle. Moving to the 10 inch tires will increase the size of the contact patch and that increases the amount of resistance the tires will have to turning. Keep in mind that the OEM steering components were all sized to the original weight of the tractor and to the expected usage.
When Case developed the 600 Series loader tractors, the only steering component they used from the garden tractors was the steering wheel. The front axle, drag link, tie rod, quadrant, spindles, bearings and rims are all specific to the 600's because all of them are beefed up considerably. Yes, I read your post and I know that you believe that you have this covered but now is the time to make sure you get it right the first time out of the box because redoing it is costly, time-consuming and discouraging. If you intend to use the Yanmar unit, then I would suggest that you change the steering quadrant to the one used in the 600's and go up one size in the tubing used for the drag link and tie rod. I think that using Heim ends on those rods would also be advisable. If Yanmar says their steering unit works fine with a 1.5 GPM oil supply, then you can spec the pump that way. I just want you to be sure that you have decided on the Yanmar unit before you order the pump because most other full-time power steering units require more than 2 GPM to work flawlessly.
It's too bad that Case never worked with Ertl or any other manufacturer of scale models, to produce garden tractor models so you could stretch one of them to see what the end result would look like. Going from a 48" WB to a 60" WB is a substantial change to the appearance of the tractor.