Barracuda spring sag in rear.

IMO, most of the suggestions here are band-aids. My recommendation is to replace the springs. Yes, it's not the cheapest option, but it is the most secure. There are plenty of vendors out there. I went to McVeigh Truck Springs for the set I put in the 73 on the recommendation of another FABO member. It solved the sagging problem and did not adversely affect ride quality.

My reservations about add-a-leaf and coil-over shocks are these. Both stiffen the rear suspension which makes the car understeer more. The coil-overs place more stress on the upper shock mount than they were designed to take which could lead to failure.

How big a tire can be mounted may depend on whether or not you move the spring inboard and how long the axle is hub-to-hub. I run 15x7 with 4¼" BS. The most I could get under the wheel wells on a 72 BBP disk brake Demon was a 235/60-15. It's very close to the outside edge of the wheel well in front and close to the spring in the back.