DAN02 -
With all due respect, in this instance, TOOLOW91 is probably correct. MM's are made from 2 x 1-1/2 x .083 tube and it appears the other parts are stamped out of .083 commercial quality steel. Most likely HR pickled and oiled - not cold roll. HRPO is less likely to crack than CR. In applications like this "one guy's metal" probably (literally) IS another guy's. There's only so many places that one can buy the raw coil steel to produce the tubing. I'm going to say that I'm 99.9% sure that the tube is mechanical grade ASTM A-513 and the other parts are ASTM A1011 (commercial grade). Regardless of the steel's point of origin (be it domestic or foreign), it has to meet these specs. If the parts were produced from a different alloy, you can bet they would be marketing it, because they would have to justify why their product is 3x the price of their competitor. The difference in quality from one sub-frame to another is going to be based off of design and steel thickness.
I'm a total moron when it comes to being a mechanic, so please feel free to correct or give me a hard time about anything I post that appears to be wrong. (I have pretty thick skin). I spent over 20 years in the steel business, so this is a topic I know a little about.
-Kevin