It would help if you told us what the models of the cameras were.
The front cams look like Panasonic DVX100 cameras, while the back looks like an FX1.
I would bet that the HD cam was 1.2 pixel aspect ratio while the SD cams were 0.9. The difference is probably somewhere in the conversion. I would personally say screw it, and zoom the HD angle at like 105% or whatever it takes to eliminate the bars, and then matte anything that spills over.
And if you're doing this in Final Cut, can't it work with multiple formats in the same timeline? Why the need to downsample?
Great footage though, nice work.