Yes, all Sierra diffs are the same shape. (There's the bigger 7.5" diff, but I think it was fitted only to Scorpios). By far the most common ratio is 3.62, but there are also ~3.38 and ~3.9 versions. I think the latter was fitted to diesel cars, and the former to the more powerful DOHC versions. Usually people don't pay much attention to the diff ratio and just use whatever came in the donor.

Search around the forum, questions about steel suppliers comes up quite often. The lengths required were also discussed a couple of times. I used 42 m of 25x25 and ~14 m of 20x20 (19x19 unavailable here).

I will just use what ever comes in the car in that case. If it came in the car its good enough for this :)

Thanks for your measurements. Maybe if I order a little more I should be okay. round it up to the next 10 or something.