Welcome. I have never seen that swap completed on a first gen. Someone is always 'working on it' and they never finish. How does it run? I guess they kept the computer as I see fuel injectors.
The rear for our cars is pretty hard to find and expensive. The S10 is a leaf spring rear so if the spring perches and spring eye distances are copacetic, it makes for a smart swap. Disc brakes are just the icing on the cake. I know second gen F body rears will fit but need to adjust the perches for the proper fit. They are getting scarce too. I would love to see pics of that rear in place and how it was done.
The lug nuts will probably be m12x1.50. GM switched to that size many moons go.
Be glad you have 4 wheel disc brakes. That is what all cars should have, yet many don't. I have a 1979 Trans Am rear in my '68 just to get 4 wheel discs. I hope they used a 4 wheel disc master cylinder and combination valve.