there are a few sites out there like that. I am doing the same over time. To many good ideas that I got from other folks not to share. That is how we make this site better. Daze is very good to deal with.
On the pipe vs. sqr tubing. There is a company that is making there subframe connectors from round tubing. They look really nice to.
I guess it depends which way it would be flexing. I guarantee that steel, if bent diagonally, is far stronger than the round, but the sub frames won't necessarily be twisting like that. I'd use square, myself.
Unless you have a tube bender. I used a tcp subframe connector as a template for mine. Due to my side exhaust, I needed a funky bend in it. The guy, who did it, had a pipe bender and did it in one piece.
Granted, but that's a pretty big "unless" for most DIYers.
I built Day's connectors as my very first welding project. Here's a couple pictures of them done. Here they are installed, before I painted them.
Then I decided they oughta be welded to the floor, so cleaned all the paint off, and so far have the passenger side welded, but not cleaned up. My welding isn't very pretty yet, but it looks like it should hold.