Lol! I don't have personal hands on experience but I'm kinda weird and enjoy random car factoids like this. Most of my info comes from forum discussions, websites with GM car specs, and resources like wikipedia, all of which could contain misinfo, but everything I've found on the longer Monte chassis seems consistent.
Notice on the Monte's they have that huge like 8 ft. long fan shroud, apparently because the engine is "set back" further in the longer frame. I always figured the 4 door and wagons were longer in the mid section for the extra doors? Everything I've read says the Monte is longer in the engine bay, so the question between years is if the fenders and hood transfer between the first 3 years? That's good indication of whether the frames would be the same.
Funny, on later 78+ G bodies the chassis is basically the same for 2 door, 4 door and wagons, but the El Camino is quite a bit longer wheelbase. Most people think wagons are longer but they have the 108" wheelbase, just like a 69 Camaro.
As Blake said GM has done some strange things over the years. Good thing Speedtech has a frame to help get it right, and in an ExtReme way too!
