I had the one piece ones too, they came with the car when I bought it. Yeah, it sucks to replace them and you almost have to have the toe boards cut out to do it. I had the toe boards cut out of mine and then you have to either bend the flange in the frame or bend the torque box to get the box in (and you'll need a hammer, dead blow or rubber mallet). Once you get the box in there bend the flages back and put it together. Sheet metal screws always help with assembly, don't be afraid to use them. And when you remove them they make excellent welds. I remember one or both of mine didn't fit well against the rocker either. I just massaged it with a small hammer until it was tight. All the visible parts of my torque boxes look "right" now that its assembled. People expect old cars to fit together nice and perfect but they never do, every one is different. That's why he good restoratoin shops get the big bucks.
