I put Progressive Fork Springs and 412 Shocks, with Heavy Duty Springs (for two-up touring -- otherwise, use the standard Springs), on my '79 K. I love them! They make the bike ride like it is maybe 10 years old, not 30. The difference was exceptional. Expensive? Yes, but well worth it for me. YMMV.
I have the same upgrade for my 1993 Kawasaki Voyager -- waiting for Spring to install them. The OEM shocks and forks are air, but putting air in, and maintaining proper PSI, is more hassle than I want to deal with. I've read comments from other Voyager owners who went with the Progressive 412's: everyone loves them; they are much better than OEM, and no air to deal with (most owners feel the air option is not worth the effort, or expense). Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII