more help needed seats in a series 3

hi need help got a series 3 swb which as a small rover steering wheel on
also had to scrap my disco but seats where in great nick from xs
but here the big prob im that big boned (fat)there's not room taken all mounts of bottom of seats is there any way to lower seat box (easy way)
would a 90 box fit in series is it lower
thanks for any help paul

You can get these so the seat will go back more for large chaps

have a look , will it help

The above is probably the best plan - but - i just took the seat mounts off the bottom of my D1 seats and bolted the straight to the seat box...