My guess would be that the car rotors on your truck are probably off a big late 60s or early 70s Chrysler (likely both of those are the same part number). Those were probably the same rotors originally installed as an optional disc brake set up for the 1 year only brake option on your 72 truck chassis. The standard 73 disc brake setup was probably specifically designed for the truck chassis and may, or may not have used the same rotor. The difference being; an optional disc brake rotor for a 72 only truck may not have a part number listing, or the number could be superseded to the car rotor, very few 72 disc brake units may have been sold. When the disc brakes became standard on the 73 truck, every 73 truck rolled off the assembly line with those rotors.

Disc brakes were still a relatively new thing in the automotive industry in 72 (1st introduced into production in 65 I believe, by Chrysler). Since the truck chassis was also new in 72, it would follow a Chrysler tradition of using off the shelf parts, if at all possible, as an option on a new design chassis or a body change on a product line.

I would look into the idea the car rotors on your truck are from a big Chrysler, but they also might not be available. Maybe the guy above can check rotor numbers for a 72 pickup, against the rotor number for a 70 Chrysler New Yorker. Might also want to check the rotor number on a 73 Dodge 1/2 ton light duty pickup as well. As a side note, Chrysler upped the size of a lot of car brake rotors in 73 across the product line.

If you have access to a light duty 70s era 3/4 ton brake rotor, I think I would see how it fits on your spindle, and check to see if the caliper will still clear the rotor. You may also find that the 70s era light duty 3/4 ton spindles and brakes will bolt on to your ball joints and tie rod ends. Gene