The only way to comfortably ride on the highway at 60+ unfortunately is to use a different bike, even if you lower the engine speed enough to a reasonable point you'll be at a serious lack of power and probably won't even have enough power to turn 5th gear at 60 unless the land is flat and there's no headwind.
But if you love your TW and you insist, it depends on where you live and how much you want to slow the engine down. First of all I would recommend going from 14 in the front to 15, and if you want to drop the engine speed more from there drop the rear.
With stock 14/50 you're running roughly 7600 at 60, and yeah it sounds like it's gonna blow up
If you drop it to 15/50, you're running 7200 at 60. Fair drop, but not much.
If you can afford to drop it lower, the next step would be 15/47. This lets you run 60 at 6700, which is 900 lower than the stock bike. I have this ratio, and personally 6700 is way too fast for me to run without worrying..
Next drop would be 15/45, which runs 60 @ 6400
Or you can drop to what owtcast posted, 15/42 which runs 60 @ 6000.
I highly don't you want to go lower than 15/42, heck I don't even know if you want to go that low. I have 14/44 equipped which is equal to 15/47, and I'll tell you my bike's 5th gear SUCKS. It has no power at any speed, and while it WILL climb from 45 mph to 65 it takes a long time. Like maybe 1 mph per second if you're lucky..and that's on a flat with no headwind! With head winds I've had trouble even holding 60 mph in 5th, it only holds 65 on a flat on a good day.
If you will be riding mostly flat roads I'd recommend 15/47 or maybe even 15/45, I just don't know about 15/42...you can try if you want as sprockets are cheap & easy to do. If you have lots of hills to climb up maybe try 15/50, it won't drop the engine speed much and 60 will still be pretty high but you can't afford to lose any more power.
But like I said it all depends on what feels too high for you. some people run 14/55 and run the damn thing at 60 still which is over 8000, I can't stand to go anywhere near that cause it makes me feel like it's gonna blow up in no time and leave me stranded. I like my bike best with 14/44 running 45 mph @ 5000, sounds perfectly relaxed and happy. If you can stand to run it at 7000 though, I recommend 15/50. If you are like me, you'll wanna get 15/45 or lower.