I used to work at a company that cast parts for gas turbine engines (Jet engines), medical implants, and other similar applications. Many parts were made from stainless steel. There are many different types of stainless steel.
Steel gets it's strength from alloying with carbon. The problem is higher carbon steels tend to rust easier. If you want to make inexpensive stainless steel, you take out some carbon, and then you need less of other rare elements, like chromium to make the steel not rust as easily.
Some of the "stainless steels" I worked with had a tinsel strength of over 100,000 pounds per square inch, at 1200 degreed F.
You can get weak stainless steel bolts, and you can get strong ones, for more money.
I'd be interested to know about the 720 outboard idler. I'm not up to any fabrication for the time being. But I am replacing this 100A with a 60A for a Subaru Loyale. It's smaller than the 100A but still larger than the stock 35A and the current size belt has it pinned against the radiator hose at the water inlet. I'm thinking a longer belt could help. I'm also curious whether my engine mounts are too wore out; whether newer mounts would get me more clearance above the Idler Arm. I'm sure I can get it running for now but I'm worried about conducting heat from the water inlet, as well as having that hose split directly over the alternator...