Barracuda spring sag in rear.

Adding an extra leaf is proably the quickest, easiest, & cheapest way to help lift a corner of a vehicle or prevent sag. Re-arching will help as well, but it's going to be unpredictably temporary depending on the condition of the springs. Try to find a set of springs off a similar car and take the 2nd or 3rd leaf out and add it to yours. It should help an will last the longest, tying you over until you're ready to buy some new ones.