Ok, sorry for the confusion. When you said arch I assumed a curved arch vs what you show in that
image which most people
call a goalpost. Straight verticals and strait horizontals. Again the
corner block orientation isn’t as critical, as long as the
bolt plates match (such as James Thomas 12”.)
In regards to your other question about a
truss that uses a forked style connection and tubes to form angles, there are a few items to consider.
1) they are not structural, meaning each side of the angle must be supported.
2) when supporting a vertical load, diagonals on the top and bottom and not on the sides is a no no. Stop. End sentence. So if using the tubes to make an angle forces you to rotate one of the
truss spans so the diagonals are on the top/bottom the that is wrong, and should not be done.