I don't know how the front of the chassis is setup but I would have drilled right through the rail and put a crush tube in with a long bolt through the whole lot if possible. Just doesn't look to have too much meat in the bottom alone is all. Rear isn't too bad but will only be as strong as the join to the chassis again I'd expect.
Yeah, I would of liked to do as you mentioned as it was a prick at the front. The bumper bracket (The bit which was ground off) is welded onto cover the front end. The bottom section which I bolted to does have 2 skins much like 2 "C" rails, all of which has factory welds everywhere to hold them together..
The only access inside the chassis is what you can see, without grinding away the whole lot. Had to sellotape the nuts onto a spanner to get them in there, as was not big enough for my fingers.
On the flip side, the other rail is even worse! has a factory tow hook, with more steel welded everywhere, over the end, underneath and the sides to support it.
The rear towbar, has 2 bolts going through the whole chassis, and the middle bolt has a captive nut welded inside. but was unsure about the thickness of the steel I was bolting the hook to.