Looking good. I took a look at those plans and loaded the dimensions into autocad this evening to see how I liked the plan and also to see how difficult it would be to adapt it to a frame mount for my tractor(don't want to pay $3.5K+ for a hoe) One think I don't like are the cylinder tabs in the middle of the boom sidewall. The tabs have a pretty good footprint at 1" X 3", but it is placing a lot of side or shear load on that quarter inch tube wall. If I build it, I think I would add triangular side plates that come up alongside the tabs and use a longer pin that engages all three tabs. This will spread the load to the stronger side walls. Back at the bend in the main boom, I think I would incorporate a larger triangular or diamond shaped plate on both sides that did this as well as gussetted/reenforced that welded joint. I see you added side reenforcement plates over that welded bend. I also see that you went with a shorter boom lift cylinder and moved the attachment down below the bend. I think that will put more stress on that bend joint. Did you do this because you already had the cylinder, or for some other reason? Unless you are trying to pull stumps straight up, I guess all that bend has to really do is lift the dipper and bucket and maybe break a bit of soil upward at the near end of the ditch. That different leverage point closer to the boom pivot point may lower your available boom lifting force.
With a frame mount, I think I can incorporate a lifting mechanism for transport using my 3PH lift arms. I have nearly 5.5GPM available from my pump so that is a little over 5 seconds to fully extend a 3" X 16" cylinder under light load.
DId you make any mods to the swing cylinder configuration from the plans? Playing around with the dimensions given for the pivot and cylinder mounting brackets, it looks as if it will be able to swing quite a ways to the left(80-90 degrees) when the cylinder retracts, but only about 54-55 degrees to the right before the cylinder rod starts to interfere with the pivot point.
Please keep the pics comming, I am very interested how it works out for you.