Welding brackets on tractor frame

[PILOON]

Typically, holes drilled into the center 1/3 of a 'beam' will minimize loss of strength if all forces on it are linear. They are not in a frame or ROPS. I wouldn't drill or weld on either.

Brackets may be attached under existing bolts, say that hold on a loader tower etc. Two-pc brackets can be designed that wrap/clamp around something, frame or ROPS, loader arms, etc.

Well said.
Ever have a close look at aircraft wing spars? They are full of holes like you point out, never near the edges or in web and always centered.
A good clean de burred hole in a web center is perfectly safe.
On vehicle frames you'll find tons of holes generally for options.
Frame welding is a big no no but bolting is accepted, in fact some truck frames are extended by bolting methods.