Ok, short story: Done. One for me, one for farmbOy and his is on the way. Here's a shot of the tapping process, just for completeness:
Now, longer story:
As discussed last night, I removed the existing cover to check the o-rings. When you get right down at eye level with it, there's not really a lot of clearance between the block and the filter housing. It would probably work, but accessing the filter in the future would be a bit of a PITA:
So I decided to add a third port. Despite what I said last night, the top or bottom are NOT an option -- at least, not for a 3/8" hole. There is not enough clearance between the mounting bolt holes and the port. I'll go back and edit last night's comments in a minute.
I did have enough meat to add a port on the outside face, but only because I made the new block 30mm thick. Any thinner and there would not be sufficient thickness to thread -- well, you could, but it's the end 3 threads that seal against leakage, and they would be cut off in thinner material.
The existing cover is 25mm thick with a 1/4" plug in the end. The drawing does not specify the thickness of the new cover, so I made it a bit thicker, just because that's what I had and I didn't feel like milling off another 5mm.
So there are 3 ports, one of which will have to be plugged with a 3/8" pipe plug (not included). But it's more flexible than Kubota's version.
Anyway, back to the o-ring issue; the WSM drawing is correct, the parts diagram drawing is wrong:
And here is the finished result: