Interesting used of rail anchors on the pusher slide.
I have a 28gpm pump on my splitter, but really, I would probably just stick with the 22gpm. You can run 28gpm thru 3/4 ports, but it will create more heat. It would be hard to guess the capacity of the control valve you already have. Looks like a old gresen, probably off some old piece of equipment. Most are rated for only 25gpm. The return oil flow, even using a 22gpm pump, will be a lot more than the 25gpm rating. Again, it will work, but the restriction is only making more heat. The 18hp engine will pull the 22gpm two stage pump easily.
As for regulating the return stroke by adjusting the detent, wont work and isnt designed to work that way. The detent relies on pressure build up in order to kick back into neutral. The detent pressure has to be adjusted just high enough for the cyl to completely retract and the moment the pressure starts to build as the cyl bottoms out, the detent will kick back into neutral. To limit stroke, you can use something to stop the cyl from reaching full retraction.They make sleeves that go on the cyl shaft just for that stiutation. You can also drill a hole in your beam and insert a bolt for the slide to hit as it retracts, or fashion some sort of bolt on stop. Anything solid that will provide a solid stop for the slide to contact as it retracts.
Making a pump mount for a vertical shaft motor isnt that hard. There are 3 or 4 different bolt circles for the base of the vertical engines. You just need to find what size base your engine has. The one thing about the bolt pattern is that even tho the bolts are on a certain bolt circle, the bolt holes are not always evenly spaced. I believe that Surplus Center has a chart in their technical section that gives the boil circle and pattern for various small engines. Find the pattern that fits you engine and lay it out on a piece of metal and start drilling holes. A pump mount will also have its own bolt circle. Just be sure to use the same bolt circle center point when laying out the motor mount and the pump mount. This will ensure that the pump shaft and engine shaft run true to each other. Dont worry about the pump being vertical, it doesnt care it it lays flat or upright, it will work in either position. As for the plate you make the mount out of, if you use a heavy plate, like 1/2", you will be able to thread the bolt holes and have enough metal to bolt everything together without needing nuts and lock washers. Not necessary for sure, lawnmower decks are pretty thin so nuts and bolts do work. Decks do rust out and break in a few years. Just something to think about.