It's vastly cheaper to make your own wedges. Worked on a timber wolf processor for a guy that didn't buy the optional 6 and 8 way wedges when he bought the processor. The 8 way was a messy joke, he ended up sticking with a 6 way. They arnt hard to make if you have the time, steel and are decent at welding. Box wedges are great, but very messy. I'm not really a fan from the waste end of things.
40 gpm will be hard pressed to find an auto cycle valve or auto return valve that will flow that much. 30 would be more like it, although I'd be running a stacked pump if I was going that big vs a traditional 2 stage pump. (Only saying this because you'll be extremely over powered using most common automotive engines.) That would alleviate the need for a priority valve. Would possibly be a bit more expensive, but simplify the hydraulics a bit.
You'll need to find out max working pressure for the cylinder you're using as well, that will basically determine system max pressure, unless you want to add a bunch of different relief valves for other accessories. All that dies is add cost though.
Think about every thing you want to run off hydraulics now. It's easier to design for them now, then try and add them on later. Trust me, been down this rabbit hole a few times. If you want a lift, make provisions for it now, want your arm/crane thingy plan for it now. I'd advise a sectional valve over a mono block so you can add spools as needed and not need power beyond or other such things that add complications and costs.
Also plan for a oil cooler and plenty of oil capacity. I'm running a 5x24" cylinder and I can attest it gets plenty hot with the 12 gallons of fluid capacity I have on hand. I was going for a compact design this time, but version 2.0 will have a dedicated cooler and a lot more capacity.
Something that helps with cycle time and keeping the fluid cooler is a quick dump valve for the return stroke and having properly sized lines/valves rated foe the flow you intend to run. Another thing I wish I did a bit differently on my splitter when I built it. Ran the std -8 (1/2") lines to the cylinder from the valve, I really just wish I would have went with the -12 working ports on the valve and had larger ports installed on the cylinder. A quick dump valve would have been nice too, instead of forcing all thay fluid back through the valve.
I'd also suggest mic channel over I or H beam. Much stronger web and doesn't have the stupid slanted profile like I beam does. Makes life easier and it's more akin to having a double beam without having to fab it yourself. Also doesn't have to be near as large as a comparable I beam for strength. I have 8"tall, 4"x1/2" legs with a 1" web. You need to go much bigger in I or H beam to get a web that thick. I got it from a local scrap yard for $0.46 a foot. Will work out much better even using 2 lengths of it vs the 12" I beam I currently have in use. The 5" cylinder @2800psi (relief pressure) flexes the beam pretty good. Hasn't ever broke and I sedomly get into something that makes it hit relief, but you can see the I beam bow. Thats also with a single wedge, it's worse with my 4 way. (Also not adjustable, wedge has to be adjustable!)
Just a few thoughts with my screw ups. Possibly help you prevent some of yours.