The 64-7, definately can handle +P, and in quite high dosages. That's a late-model all-stainless K-frame...it's *almost* as strong as it's close cousins, the 65 and 66...both 357s.
I wouldn't shoot steady diets of the HOTTEST 38+Ps available, like those wild Bufallo Bore beasties (158grains @ 1,000fps from a 2" barrel, anyone ?).
I'm not familiar with the 67...? Somebody will be along shortly who might know. If it's...hmmm...I think around mid-80s or later (not sure though) they had beefed the whole K series to serious +P spec. (Earlier could handle at least minor amounts of 38+P, even going back to REALLY early M&Ps of 1920 or so...)
NOTE: I'm not an S&W expert by any means. I'm 95% sure that a 64-7 will be new enough to be well into the +P era but even then, get a second opinion.
I think any steel frame S&W revolver made after WW II would be fine with +P which isn't all that much hotter anyway. Back in my youth we loaded Colt and S&W revolvers WAY beyond factory +P levels and never had any trouble. All the warnings are really for the old junky guns from Spain and Brazil.
The model 67 is the stainless version of the model 15. It is O.K. to shoot +P ammo in the 67. My first duty weapon was actually a model 67 and I liked it so much that my home defense gun is a model 15.
Both of these guns are K frame guns, and it should pose no problem to shoot +P's in any model number marked (post 1957 or so) all steel K frame (as opposed to the smaller J frame) gun wihout worry. FWIW, the M64 & M67 are the stainless versions of the M10 & M15. Both should be great shooting guns that will last a lifetime of +P shooting.
How are model 64 and 65 different other than the 38 vs 357 markings on the barrel? they look "almost" identical from what I've seen. The only difference is that 65 has carbonsteel insert in the cylinder, and also the force cone is slightly shorter in 65 to accomodate the longer 357 cartridge.
In the grande scheme of things all Smith & Wesson stainless steel revolvers are a relatively new invention and are therefor suitable for +p ammunition. Even the original stainless steel handgun the venerable Model 60 only dates back to 1965 and can easily handle +p in moderate doses.
I know of absolutely no differences in the Model 64 & 65 frames. Only the barrels and cylinder assemblies are different. S&W batch produced the frames and really didn't decide as to which would be which until the production run was started.