Those baseplates look pretty thick to me - 6mm perhaps (or 1/4 inch if done in the states?). For us, you need only 3mm minimum.
It would, as you say, be a shame to scrap an otherwise good cage just because of this - but I wouldn't fancy trying to grind those weld beads off if it is 6mm thick.
However, after unbolting the cage, it may be possible to cut around the plates with a cutting disc, thus removing the baseplate and the piece of floor it's welded to, leaving you with a hole the size of the current baseplate. Then you simply make up a larger baseplate to MSA spec, in 3 or 4mm, drill it for your bolts and weld it over the hole - as it will be bigger than the hole you have, it will cover it entirely.
As it looks like your bolts go into a blind box section, you'll probably have to weld nuts to the underside of the plate before you weld it to the car. Incidentally, it looks as though your current bolts may only be about 8mm (or 5/16 inch), which isn't really sufficient - so while you're at it, drill out the cage feet to take at least M10 (or 7/16 inch). But you only need three for each foot (two for backstays), so you could leave the awkward looking one that's under the cage bars, or just replace that one with the current bolt size if you can't get a drill into it.
Hope this helps! Good luck.