If they are both off non radial bikes it will be a straight swap, if the bike has radial brakes and the brembos are from a non radial bike you will need a 2.5mm spacer behind the discs. If the bike has non radial brakes and the wheels are from a radial brake bike, you will either need to machine 2.5mm off the wheel hub or from the brake caliper mounting face.
Hope this helps.
Mark