I am parting out a '75 Riviera and noticed that this car has a boxed frame,like a convertible would have. I have had a few of these cars 71-76 and never noticed the frame being boxed on any of the others,including a '73 GS Stage 1. There is also extra bracing for the rear suspension,sort of like A body 4spd cars have. It does NOT have tow package anything on it. The body is really rusty( a PA. car) but runs very strong. The frame is solid and I wonder if maybe I should keep the frame for future use. I am stripping EVERYTHING from this car before I scrap it out.
All the Wildcats I had (65 4dr h.t.-67 2dr h.t.-68 4dr h.t.), and my two Electras (75 4dr. h.t.-76 2 dr coupe) had boxed perimeter frames. Most LeSabres didn't but some did (convertibles and those with the police package had the same frame, at least in 1974). I think that all 71-76 Rivs have boxed perimeter frames too.
I had an accident a few years ago that required a new frame for my 71 Riv. I found a 72 Riv frame. Neither is boxed in the middle under the doors. I did notice however, that my 71 frame had extra reinforcements on the lower front A-arm mounts.