The B12 rim is gonna be 1 in wider and will probably fit. How wide a tire is the question. The limiting factor is how wide will the swingarm allow. Also, you may run into problems with sprocket/chain alignment as the rear sprocket usually sits out further on a 5.5 inch wheel.
You are not gaining anything by going larger on a B6, Unless you hang a B12 motor on your bike, you have plenty of rubber. A 4.5in/160 rear combo is healthy for a air/oil cooled 600.
I am putting a 4.5inch Katana 750 rear on my 1983 GS750ED. That size rear will handle over 100hp on a 750cc hot rod.
Buy premium rubber instead and save the headaches.
Ride On, Ed.
2005 GSF1200SZ
1983 GS750ED