[undertow]

That's pretty advanced. Mine has no rust there, but a bit in the seams underneath where the chassis rails and stiffeners etc meet the floor pan. I also found rust on the fuel/brake hardlines underneath, fortunately it was just surface rust and cleaned up fine.

IMO Ford did a poor job with the painting and/or galvanising on the AU. I poke around the self-serve wreckers a bit and see a fair few AUs with at least surface rust underneath, but I'm yet to see a rusty VT-VZ commodore.

I am not sure how much of the BA will fit, I think the doors are the same, but the A pillars (lower and upper) were strengthened on the BA, front rails and floorpan around them are very different (BA rails continue straight into the floorpan area rather than dipping under), trans tunnel area was reinfornced too.