Hopefully some of this will be more salient. The Model 60 and 640 had their frame window lengthened to allow the longer cylinder which would chamber 357 magnum starting with the -9 in the 60 and the -1 in the 640. However, the original steel framed guns (the Model 36, and pre-9 Model 60s, pre-1 640s) were still plenty strong when just using 38 special. If the concern is about durability on a pre-9 and a post-9 Model 60 gun, you'd have to shoot both of them an exhausting amount to begin to fatigue either of those steel frames. I have an original Model 60 no-dash (non-magnum and small window frame) and a magnum chambered Model 640-1. The 640 is larger and heavier, but neither of these features is much of a benefit on a carry gun, and when shooting 38 specials both are plenty strong enough.
I had a 442 Magna ported gun that had over 3000 rounds through it before I sold it off and it stayed very tight, for what it's worth. This included regular practice with Gold Dot 135 +P carry ammo, as well as lots of range-grade ammo. This was a traditional alloy framed gun that was not rated for 357 magnum (38+P only).
These days I carry an M&P340 if I want to pack a J frame around (Scandium Frame, Steel cylinder). I haven't shot the M&P340 as much but it has had 200 rounds of 357 magnum 158 grn, 20 rounds of 357 magnum 142 grain (hot Fiocchi), and by my last count around 1500 rounds of 38 special ammo (both new and my reloads). The gun is still very tight and the PVD finish on the cylinder is wearing very nicely and does not show a silver turn line. It is NOT enjoyable to shoot 357s in this gun. Speer makes a 357 135grn Gold Dot specifically for short barrels (same bullet as their 38+P loading), this is loaded to mid-range 357 levels, its more tolerable but I'd still prefer 38 special +P.
Regarding the build characteristics, this is why I mentioned the 38 to 357 conversions. The newer J frames are plenty strong. I don't consider them enjoyable to shoot with 357s however, they stay very tight even with regular doses of magnum ammo. I would never recommend converting a factory-equipped 38 special J frame to 357 magnum, nor were you asking about it, but the fact that other people do and the guns continue to sustain this process seems to indicate that they are quite resilient. The only consistent thing that I have seen is that the new-production Steel and Scandium framed J frames chambered in 38 special still have the same 357 magnum-windowed frame. All reports on various forums seem to indicate that these Steel and Scandium frames are identical, regardless of which cylinder they are equipped with. Both should prove to quite durable.
One more tidbit on build characteristics/durability. S&W made a short run of small-windowed 442 alloy framed guns with alloy cylinders (not the steel that they normally use). These fired 5000 rounds of +P ammo without a failure. Starting in 1996, S&W changed the 442 to the magnum window frame and also rated them all as "+P" guns. Even though the older guns have proven durable, S&W added the larger sized frame to the entire J-frame lineup, seemingly a shift to save production costs as much as a potential gain for durability. As long as you're sticking to a steel framed gun (such as a Model 60 or Model 640), I would not be concerned with durability, even if you plan to occasionally practice with +P ammunition.