bigger rear disc in rear than front a problem I assume?

Current tires are 235/60-15's rear and 185/60-15 in front on steel cop wheels, but the tires are around 10 years old and will need to be replaced (and may change sizes) if it ever drives again. I have been thinking about minitubbing the car and going 15x10's in back, but decided to put that off until the motor swap is done (the reason I went with the hdk k member, hence the need for mustang II brake kit).

I appreciate all the help as I need to install the k member/brakes/coil overs before I start mocking up the motor tranny