i doubt it would be a simple install. the R51 uses floating calipers and the Z33 uses fixed calipers. you would need to change out the caliper mount and i don't think Nissan designed any interchangeability into them, considering they're on completely different platforms.
apart from getting the calipers themselves to simply fit, you'd have issues with your master cylinder and its effective braking capacity. a R51 uses a 2-piston caliper and the Z33 Brembo uses 4 pistons. the larger fluid capacity of the Z33 would give your R51 a very deep and soft pedal feel. you'd probably have to upgrade your master cylinder and/or brake booster to match it to the capacity of the Brembo calipers.
i'm afraid what you may have seen was one of these kits:
http://cgi.ebay.com/ebaymotors..._1165
they are "Brembo Caliper Covers" that simply glue onto the body of your existing caliper to give the look of actual Brembo calipers.