I have Alfa 147 seats in my kit car and when I got mine the existing Alfa subframes were different, I assume because the bottoms of the seats are slightly different shapes (mine are anyway) but I don't know why.
I removed all of the Alfa stuff and bolted some metal box section directly to the seats - the mounting holes are in the same place on both seats so once I had the box section fixed I could mount them to the floor of the car. Sorry not much help other than suggesting you do away with the existing subframes and start with something else.
Also IIRC the Alfa subframes were quite "tall" so will make the seats sit quite high in the car, assuming you are going to try to somehow bolt these to the mini directly; you might lose a bit of head room (of which there isn't a great deal to start with) so another reason to try to make something a bit "lower-profile"