I'm new here but I'll throw in my 2 cents. Your subframe section has the steering box inside the frame rails like a car. Full size GM trucks have the box on the outside. The S10 also has the box inside the frame, but the rails extend straight to the end. Your subsection has a turn out where the original frame horns are joined to it, which pretty much indicates it's from a car, but which model? The best way I can think of is to measure everything you can: track width, crossmember, control arms, spindles, steering links, holes in the frame, etc. and bring your photo and measurements to the local pick a part. Look at Camaro/Firebird, Nova/Ventura first, if those don't match try Chevelle, Impala, Buick, Olds, Cadillac untill something matches up. I know you were looking for a more definitive answer, but that's what I'd do anyway. Hope this helps.