Good stuff here. I agree, 350 is by far, the most popular of the small blocks, and for many good reasons, most of which have already been duscussed.
The original poster asks which is the "strongest" small block. If he means "physical strength" (as in resistance to destruction), 350 most definitely. If he means "most powerful", 400SB (with modifications) is it for obvious reasons.
On the issue of "rod/stroke ratio"... Many high-end engine builders have their own "ideas" about this. Smkey Yunik wanted "as much rod as you can fit in there!" Most agree, 1.7:1 is "ideal" for most performance and drag applications. By no small coincidence, this is the r/s tratio of a 5.7" rod and a 3.25" stroke (327). Hard to argue with the higher rev durability of 327...
A misconception about cylinder pressure and "dwell " is present. "Dwell" (the time the piston is "at rest" between direction changes) is longer with a higher r/s ratio, INCREASING cylinder pressure, not reducing it. It can actually increase the potential for detonation. But it DOES make more power if not detonatng. Added dwell at BDC also improves cylinder "filling" on the intake stroke.
We use 6" rods whenever possible for the 3.75" stroke. Sometimes, budget constraints prevent it. Circle track 350s "like" 6" rods on longer tracks, too.
For ease and availability, 350 is the better choice, IMO.