[Wasted Income]

Plates: My trailing arms have those plates as well........but I'm not sure what they came off of.

Holes: It is possible to drill holes (reasonably sized) in THE EXACT CENTER of a beam only to lose a negligible amount of stiffness. A good example would be the holes that are sometimes put in floor joists for plumbing and electrical conduit. At the exact center of the beam, there are no tension or compression bending stresses. On top, the beam is under compression, and down below, it's under tension. If you keep the hole small enough for the size of the beam, and the amount of weight it's expected to carry, you should be ok.

Personally, I'd be weary about whacking into my trailing arms with a holesaw if I hadn't done some precise calculations.......especially if it sees any towing or heavy hauling.

My $.05