Will Aspen Wagon Disc Brakes Fit On My Scamp?

You should be able to find 73-76 A body large ball joint upper control arms for around $75a set, give or take. Still a lot of them around if your not in a hurry.
The adapters from DrDiff should be good, IMO he wouldnt put them out there if they werent. I would use them if I had newer small balljoints already and wanted to do the swap without having to buy the new large balljoints and the later UCA's.
Spindles out from a F, M, J or late B body fit. Dust shields, spindles/knuckles, caliper brackets, calipers(for cores at least), rotors- if good, and if your pulling the parts get the master cylinder(core) and distribution/proportioning valve too. Save all the hardware!