Will,
You have plenty of metal in a 3 X 5 -1/2 angle. If you have the 5 inch part of the angle vertical, you should not have a problem other than it being heavy. I started out with my mill track built with 3 X 4 -1/4 by 20 feet. It had too much bow in it and was hard to get it level. I added a piece of 6 inch channel to the sides of the track to stiffen it up, it made it easier to level, but is heavy to put the wheels under it and move it.
If I had to do it over, I would start with tubing for the track frame. If you are going to leave it set-up permanetly, you could save a few bucks by using a lighter weight tubing. Also, you should take into account the size of logs you plan on sawing. If you are going to saw a 20 ft log that weights over a ton you should have more metal in your track frame.
