Interesting. It's too wide for an S-10, the shape at the front of the crossmember is not the same as a Chevelle (64-67 or 68-72), or Malibu (78-88). The spindle (pic #9) is factory, with it's spindle position and date-time casing number system, and looks like the tall style used in '70-up F, '73-up A and '75-up X cars. Only the '73-77 Chevelle used that spindle and those are definitely too wide. The frame on A-bodies (Chevelle etc.) curves outward pretty quickly after the crossmember, this does not.
Therefore I'm saying '70-up Camaro or '75-up Nova (or associated F- or X-bodies), with their straight-shot rearward after the front cross, it appears from the pics that is where the blending between the two frames was done. That width also looks about right to me.
I don't have one handy to compare, but that's where I'd look.
Edit: Since I helped, can I tease you about the blue paint?