Haven't worked on a 70 Monte in particular (that I can remember) but I've worked on a lot of "A-body" cars of that vintage and the frames all had that open C-channel in the middle. They ran the brake and fuel lines through them, bolted the trans crossmember to the bottom/inside of the C-channel, etc. I can't recall any of them being fully boxed in those areas from the factory.
@grandmasmalibu on the regular a-body cars that is true. The grand prix (69-72) had a boxed frame very similar to the convertible, I'm sure the Monte Carlo is the same.
