[NB I apologize in advance for the mysterious mix of metric and Imperial sizes:o)]
First, get 4 nuts which fit your Metro seat bolts (assuming you have them) - I think these are 13mm across the flats (about 1/2 inch AF size). Also get four flat washers about 1 inch to 1.25 inches in diameter.
Then, weld each nut to a washer like sketch A :-
Next, cut two pieces of square steel tube, (1inch to 1.25 inch square, about 4mm thickness) to the appropriate length to fit between the rear end of the Metro seat runners and the Mini rear floor. Weld two bent steel tabs to it, with holes to take a bolt, as in sketch B below: (You will need to vary the bend of the tabs to fit the curve of the floor)
Then, invert the washer/nut (A) so that the nut is inside the square tube, and weld the washer to the tube, as in sketch C:
These will be your 2 rear mountings. You will later drill the floor to fit the holes in the tabs, and reinforce the floor below either with a piece of steel (sort of large strip washer) or two hefty plain washers. However, you should first complete and fit the front mountings..............
You may wish first to remove the Mini seat brackets (2 x 7/16 inch bolts). Position the seat so that the front holes in the Metro runners are at a suitable place on the front floor crossmember. Beware the rear door pockets as the seats recline, and make sure you are happy with the position. Mark the hole centres on to the crossmember, and drill/file these out just big enough to accept your 13mm nut.
Then take your 2 remaining nut/washers, and invert them so that the nut is inside the crossmember. Weld the washer edges to the crossmember - hey presto, instant captive nuts, as per sketch D:
Bolt the seat runners in to the front mountings, bolt the rear of the runners to the supports made at C above, then drill and fix supports through floor as explained.
Repeat for other side of car, smile open beer and enjoy your new comfy seats.
Colin Sutton colin@teamflyingpig.com