Re: 425 FEL Build
The skeptic in me tells me that a round crossbar (pipe) is used, instead of rectangular or square tubing, mainly because it is easier to manufacture. I do think that the loader arm assembly is much stronger if the crosspipe goes through the loader arm and is welded on both sides, compared to just butting up to and welded against the inside, like the Johnson loaders. I think it would be harder for a manufacturer to cut the square or rectangular holes.
On my homebuilt loader, I used 2 x 4 x 3/16" tubing, cut through the arms, welded on both sides. A heavy walled pipe would have worked too.