As for it being any harder....YES.
Anytime you increase chamber pressure it is harder on the firearm.
As for your 64, shooting +P in it on a regular basis *Might* cause the the cylinder Lock-Up to become a little loose quicker than if it was standard pressure. But that would also hold true for any +P rated fire arm also. The best thing you can do is keep it CLEAN. A dirty cylinder/Bore will contribute to excess pressure.
I also called Smith and Wesson, and they told me they didn't recommend shooting +p any anything specifically rated for +P......which amounts to "a generic answer"
Can't say I blame them, that way if it breaks, they can say..."Told you so....."
I had a Model 10 and after 300 rounds of Remington 125 Grain +P through it.
I could tell NO difference in Lock up, or Cylinder/Barrel gap.
Plink with plinking ammo, and practice with what you carry.
I honestly don't see you having any problems shooting +P in it.
If it were mine, I would shoot them in it, and NEVER loose any sleep at night over it.
Now maybe your Great Grandchildren might...