Hi, I have just used these to replace my shroud u bolts, they are very good quality, I went up a size from 8 mm to 10mm. They look much better than u bolts too.
It is still worth angling them inwards towards the base of the mast so that the force acting down the legs of the "U" (or "A") is as close to axial as possible. If they stick vertically upwards, there will be a side load as you tension the shroud trying to bend them inwards.
As far as I'm aware, you just buy the closest you can get and the rig will do the rest.