Rear leaf springs are available from any Mustang supplier. Eaton spring supplies the most accurate reproductions, but they are more expensive than a standard style replacement. Since you have a '73 Mach, you have competition suspension, which utilized a 138 lb/in rear leaf. I've used the 71 429CJ spring in several cars with excellent success.
With the correct springs you won't need them. Hard telling what they were carrying that needed that extra weight capacity, but if it were me, I would check every nook and cranny, especially the drop-offs in the trunk, for leftover contraband.
Back in the 60's / 70's in the rust-belt northern region it was common to load up the trunk with additional weight to (hopefully) improve traction on the snow and ice during winter months (those darn poly tires!)....PO must have loaded up several hundred pound of pig iron from the steel mills to need those kind of extra springs and air shocks.
Another possibility is running over sized tires sticking out beyond the quarter back in the 70's. Could have used all of that suspension to get the back end up high enough for that. Either way, as all have indicated that is all added on stuff and needs to go if you want original. Also keep in mind, visually from the factory all of the fastbacks sat a little low in the rear compared to the front. A heavier weighted leaf will help level it out more. If you have the chance look at one of our cars going down the highway at speed front ends always look high compared to the back. Just air pressure on the front and stance in the back.