I've had an HD-8 for probably 10 years. I use it on my
B2710, flow doesn't matter it just speeds up raising the ram. The return does matter, has to be directly to someplace with no restriction. I was able to get a fitting with an o-ring that fits the fill opening on my transmission, it's right between the 3pt lift arms so that was easy to plumb. Every tractor is different, my neighbor has a JD which has threads on the fill port so it was easier for him to make something up.
We have a lot of rocks (glacial till) which makes pounding difficult and not very precise. The posts move around as they push the rocks out of the way. I've found putting a dull point on the post helps, even though shaver says not to do that. If you're luck you won't have rocks. If you try to pound posts with slots or holes, like for a split rail fence for example, the pounding will often split the post. I keep the post solid, I also buy oak posts from a local sawmill. When I used treated pine posts, I had some split but not too bad. It helps if the top is square/flat so the ram hits the whole surface and not just the edge.
It also helps if you have a top and tilt, it makes it easy to get the thing plumb. Mine has the manual crank for plumbing the machine, that's a real slow, knuckle busting process. So if you don't have top n tilt, get the hydraulics on the pounder.