Welcome to KZR!
This might sound weird coming from me, considering that I have an upside down front end and monoshock rear on my bike, but you need to consider what 650ed and rstnick are saying.
First and foremost, don't think that the original front forks aren't up to the task. Keep in mind that they are 32 years old and probably haven't been serviced like they should have over all those years. Almost every used bike that I have ever ridden could use a fork oil change and springs that were the correct rate for the rider. Those two things alone will make an enormous difference. Before you swap the forks, give Race Tech a call and see what they suggest for springs, oil and maybe an emulator kit for those forks.
Switching to a USD fork is not necessarily an easy conversion. You lose ride height and cornering clearance. You still need to change to the correct springs and oil. Yes they are more rigid, but they are made for lighter bikes with stiffer frames, and 150 lb riders. Also you lose some turning radius and need to make provisions for keeping those fat fork tubes from banging into your tank.
If you consider all of these parameters and still want to make the change to a USD front end, then I won't criticize you for it. Actually, I'll be happy to advise, but I don't want you to think that it is easy or straightforward. Every change begets another change, and a properly sorted bike is the result of a comprehensive suspension setup. Just adding a stiffer, and more adjustable fork will not necessarily improve your bikes ride or handling.
