If the rears are anything like that on the lasers, then there is an adjusting screw to wind the calipers in and out so you can fit the new pads.
Also best when fitting new pads and discs to bleed the system. Remember to move the pistons in and out to flush the bad from the caliper galleries.
I have an ST170, plus a GTR Familia rally car.
Will have to look at the ST to see if it is the same.
But Lucas pads are woeful on the standard discs.