IMO,
Drilling holes through the frame rails and then attaching the subframes attempting to "squeeze" the rail to keep everything tight will eventually lead to elongted holes (or rips) in the frame rail. Now to overcome that you can always drill the holes slightly larger. Weld in tubes to the frame rail and then pass the bolts through those tubes, but the reality is you are still relying on "squeezing" at two points.
Again IMO,
Welding all the way around the mounting structures is a much stronger and lasting solution as it distributes pressure over the entire footprint, and does not allow movement under pressure.
Do you disagree? Or are you just asking for me to elaborate? Or did you just miss the subject line?
Thanks,
Scott