How do you determine the overflow box gph
If i'm understanding your question correctly, then all overflow box tells you the max gph
How does it not overflow from power outage?
There's actually a few ways. Do more research. The amount of resources & effort you put into try to prevent flooding with overflow boxes will outweigh the resources and effort you put into drilling the tank.
How do you determine the return pump size?
If you have a 29g, then try to get at least 290gph pump (10x volume). The height and 90 degree elbow pipes also decrease flow, so aim around 400gph. There is no general rule, but you have to think about the sump as a whole. What will you put into the sump?