Some of what you are experiencing is likely the stability of the bike even though it first seems like it's power-related. Improvements in the front end will make the low power less of a factor, and make the bike more comfortable and confidence-inspiring on the highway.
A more modern front end makes a phenomenal difference in comfort at speed. An alternate, and easier, option is to improve the existing front end. The original bearings are usually shot by now, and are fairly easily swapped. Swapping in an aluminum lower triple tree will yield HUGE handling and stability benefits over the stock lower since that stock lower flexes like mad. Swapping the upper triple won't yield much benefit. Fresh fork oil and appropriate springs makes a difference too. There's also the rear end to address, but I bet the front end is where you'll see the biggest difference.
