I had mine done with the Ziebart Rust Proofing way back in 1974 and it has preserved the car very well. No rust through anywhere. What is the current condition of the undercarriage, have you already cleaned it up?
Under carriage is very clean. All new floor boards!
Unless you are going to use yours as a daily driver I would personally not undercoat. With that said mine still has the original undercoating/rustproofing on it. When I get around to cleaning it off I don't plan on re-undercoating.
If your interested, here is what the Mustang Club of America states in their judging rules (for concurs):
5. UNDERCARRIAGE Points
A. Undercarriage:
Undercoating: No undercoating allowed in Trailered Class.
Undercoated Cars: Everything must be covered except drive shaft, transmission, rear-end housing, exhaust pipes, mufflers, springs and plates. Gas tank, fully undercoated or have overspray around leading front edge and two sides. Must show some evidence of overspray. Convertible plate must be undercoated.
Non-undercoated Cars: Undercarriage primer either red oxide, dark gray or black with some body color and pinch weld paint overspray. (Primer paint drips should be visible for non-undercoated cars.) Convertible plate, black or natural........................................................................................................................................................ 20
Workmanship, condition and cleanliness of undercoating. (Workmanship of undercoating must be poor quality or excessive to deduct points). Workmanship, condition and cleanliness of underside of floorpans ..................................................................................................................................... 15