That IS really fast for a bicycle... what kind of bike is it? I've heard of some of the more powerful e bikes hitting those kind of speeds but those builds are usually on downhill frames or frames and forks that are beefed up to withstand higher speeds.
You could probably run balance beads in your tubes to balance the tires which would also help with stability at those speeds. One of my friends has a bike shop and swears by the balance beads compared to conventional weights on the spokes or rims.
Also, the tires need to be in perfect condition and built to go fast, most the better namebrand tires will handlde it just fine.
Of course slowing the bike down to the mid/high 40's is always the safest bet...
For me, I hadn't had any time to do much with mine the past few days, but I did score me a really cool Girvin Vector fork for it earler
And all my parts for the new carb are all finally here, mainly new jets and such to tune it in just right. So far I hadn't took mine past about 30 or so mph, but I'd like to get it as fast as yours once it breaks in all the way. I'm hoping with that new fork, better wheel bearings, and better tires it should be stable enough to safely go over 40, I'd like to gusset all the stress points on the frame too. It's plenty stable at the 30+ speed range so I don't see any problems pushing it another 10 mph faster.