Reply to "Rust Prevention Inside Sub Structures"

All the ones you mention work well. The 1/2" dia frame rail drillholes (3 per side) work well. Some then cork up their freshly drilled holes and fill their frame rails with drain oil or something similar. Let set for a few hours, then pull the corks and drain. And many of us drill an additional 3/8" dia hole in the extreme base of each leg of the horseshoe shaped stamping that holds the rear a-arms (2 holes per side).
One method that does NOT work is filling the subframes with urethane foam meant for boats. It does not completely bond to the sides & bottom of the steel rails, and post-cure shrinkage adds to the problem. So water wormholes down to the bottom of the rails and does what water does best- it rusts- even worse than normal since evaporation is now impossible. The damage must be cut out and rewelded as in your car.