Ed, the placement choice is yours. Either works but you have to remember which one so as to know the temperature level that is safe. Cummins shows after the turbo in all their specs. I had read the primary reason was if the probe happened to break it wouldn't hit the turbo wheel. The vast majority of people on the forum mount it pre turbo. I discovered the Cummins thing when I noticed they had very low numbers for recommended EGT. They recommend 700-900 deg F post turbo. Those numbers can vary depending on how close to the turbo outlet you have the probe. They recommend 2-7". Pre turbo placement can also vary the numbers. Recommended increase is 200 deg F but I've seen number ranging from 300-450 deg F. In general, 1200 should be the limit on pre turbo for any extended time. Must remember the temp inside the cylinder can be hotter than the exhaust manifold. Aluminum melts at 1221 deg F but pistons aren't pure aluminum plus you have those cooling nozzles spraying oil on the back side. I'd think 1200 before turbo and 900 after would be about the max. Just remember that probe placement can vary those numbers as well as how accurate the probe and gauge are. You might contact the guys at Hewitt Industries. They invented the pyrometer for turbo diesels back in 1952 so they would be a good one to ask. Hewitt Industries
The closer you get to exhaust ports the more accurate. Turbos extract heat from the exhaust and the amount changes on how hard they're working. So post turbo only works if you have a known safe limit for that combo.
The steel pistons were found in larger engines and are generally an all out high performance item for the 4bt/6bt not normally seen by most of us. In fact, I'm not even sure anyone is making them for the standard B series engines. They also will be super expensive.