The K505 are still made and are Dunlop's current fitment for the NH 750, but forget them. The BT45 were good tires but are getting a bit dated and the new Metz Lasertec, Pirelli Demons, and Mich Activ are all better bias ply sport touring tires.
Metzler's current fitment for the NH 750 is the Z6 radial in answer to your question about radials. I replaced the 2nd rear BT45 on my 97 GS that was more than half worn out at 2k miles with a Z2 radial that lasted about 14k miles. A set of Avon radials went about 13k miles. The radials handled better both wet and dry and were the best value in the long run. Ride better too.
I'm currently running the Lasertecs and they handle as good both wet and dry as the Metz and Avon ST radials I've run. Just won't last as long. Tires I've tried on the GS's and miles I ran them:
http://img.photobucket.com/albums/v443/ ... irelog.jpg
My last of 4 Hondas was a NH 650 I traded in on the 1st GS, same engine as your late model 750; wish they still made those NH 750's.