These are all great tips!
This is a generalization, but early Mustang underbodies were red oxide primer. Whatever body color got underneath during the painting process stayed. My 1:1 experience is almost 100% San Jose built cars, and Ford's processes notoriously varied from plant to plant. Most of the cars I have seen had pretty complete body color over the sound deadener in the rear wheel wells. Any outward-facing vertical surfaces like unibody frame rails were pretty well covered as well. Not much body color made it much further than that. All of the underhood sheetmetal was semigloss black. Front unibody rails typically have some black coverage from the underhood paint. Leaf springs were bare steel. Rear axle housings were semigloss black, but the dropout section was red oxide primer with a black bearing housing. Exhaust is bare steel, as were tie rods and the sway bar. Spindles were bare iron. Upper control arms were black. Lower control arms were originally dipped after the ball joint was installed, so the inner ends were black and the outer ends were bare. Fuel tank was galvanized steel.
1966-later engines were Ford blue. A Shelby most likely had an aluminum intake manifold. Most Ford automatics have a bare aluminum case and a steel pan. The exception in the early Mustang (69-73) is the FMX, which has a cast-iron case. Manuals (top-loader) have an aluminum bellhousing, bare iron case with an aluminum tailshaft housing. Driveshafts were bare steel.
Most of the cars you see in the internet are over-restored, but you will get ideas from looking at them.