Dwell angle is the amount of time the piston stays at TDC while the rod and crank journal traverse the arc of the circle the journal follows. Think of a dime and a quarter. Each degree of a dime's circumference is so many hundreths of an inch wide like a pie shapes slice or one degree). On the quarter, you still have the same 360°, but each degree is slightly longer in width. Meaning the pie slice has a longer "crust side". It takes the crank and rod so much time (in crank degrees) to traverse the longer peice of pie. That number of degrees is the dwell angle at TDC. This is important in terms of cylinder pressure development after the mixture is lit and desiging a stroker's timing curve. It's also why having the right timing curve while running a fast burning pump fuel is important with these engines.
However, this has nothing to do with block stress. The rod and piston, and the crank at all perfectly straight up and down at that time. Plus, the piston and rings are in the strongest part of the bore. The stress we're talking about is when the piston, rod, and crank are not aligned. That's just before the piston is at 2" up in it's travel from Botom Dead Center. The cranks basically driving the piston into the side of the wall at that point, and thats what makes what's called the "major thrust" side soooo critical in terms of sonic testing. You can have a spot in the bore as thin as .080 without a big issue unless it on the major or minor thrust sides. The major thrust needs to be at least twice that to avoid the worst case wall splitting, or power robbing wall flexing that will eventually lead to cracks.