No idea about strength. I had built my own wooden power rack. It worked like a champ - had up to 365# in it for working sets and it didn't even budge. I could do pullups in it too (weight 100-110kg). I also loaded it (and the safety rails) with 500 or so pounds, then added me and it was as strong as a house.
A comment on the J-hooks. Take a look at this video (WFS)
I used that method and it was solid. Felt very comfortable using it.
I didn't get a special cut pipe - I just packed the other side with some plywood - much like a washer. It actually had the added benefit of being a sacrafice piece of wood where it would get damaged, rather than the wooden uprights itself. It also served as something I could get a better purchase on when tightening it up. (this probably makes no sense in words - just make a washer out of plywood and you'll see)
I watched a bunch of these videos to get various ideas - because they're always something that is mentioned "wish I did this".