I would recommend
Progressive Fork Springs and
412 Rear Shock Absorber replacement. They're a bit of cash, but you can ignore the air assist (you will not need it), and they will improve the suspension performance, and safety, exponentially. They need to be done as a set, front and rear, as doing one, will make the lack of performance in the other, blatantly obvious, which means that the ride may be a bit scary, at times.
Essentially, you get progressively wound springs, which behave as two different springs. This results in a ride which is amazingly smooth, comfortable, and much safer. Your bike will ride as if it is modern, instead of 25+ years old. The wheels will track the road more accurately, which means more safely, with drastically improved traction on rough roads. Be sure to get the
standard rate springs, on the rear suspension, unless you ride two-up, 90+ % of the time -- the heavy-duty springs are superb for two-up, with full saddlebags, trunk, and tank bag... I went with the heavy-duty springs, and they are too stiff for one person, but perfect (on the lowest setting), for two-up. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII