Mike,
first: don't overthink this. The diagrams from Ronstan are very helpful. What I have is their 12:1 system.

This one is number 8 on the page you have linked. Their load rating is 990#, which seems fine to me (but I'm not a professional rigger, so any discussion here cannot be considered professional advice). The 6:1 that is part of it is rated at 880#, which means that the single block is the "weak" spot, which is as it should be (easier to replace than either boom or 6:1 purchase).
They suggest Dyneema for the cascade part, which I tend to agree with, but anything with a SWL of 990#/2 should be OK. More, if you want to account for loss of strength in splices or knots. The line in the 6:1 purchase never gets loaded much, so you can use almost anything that runs w/ little friction and is easy to grip.
(You could also do the 20:1, similar to what's shown in #6, that's what a lot of people have, but I'm not sure you need to use wire like they suggest; if in doubt, spelunk here in the forum to read older descriptions of vang setups.)
You won't be able to get 45° given the geometry on the DS. If your mast is standard length (has not been cut down by you or a P.O.) then you should be fine mounting the bottom a close to the deck as you can manage and the top about 3' along the boom. That should give you perhaps 30° if you are lucky. That seems to work out for me.
A neat method for attaching the vang to the mast is to use an eyestrap at the front (!) of the mast and to tie a loop. The purpose of the strap is just to keep the loop from sliding up, while most of the force is held by the mast itself. That means a very ordinary eyestrap will do.
For attaching to the boom there are many different options. You could use a U-bail, as you describe, but through-bolted; definitely not using screws. I'm not sure you even need to worry about using a bushing. You could use a fixed strap attached with blind rivets. That's what I use If you do, make sure to fully align the strap with the direction of pull, or it will get bent. Don't ask me how I know. The rivets seem as if they wouldn't be strong enough, but they've held, while the strap got bent. Goes to show you.
Then there's the system where you attach a slot fitting that takes a T-shaped key. Makes for a quick disconnect, but is intended for wire. So you'd need to use wire between the boom and the single block.
Whatever hardware you add to the boom make sure that any dissimilar metals are galvanically isolated (using TefGel). Or corrosion will widen any holes until fasteners pull out.