On your deal buy a good set of leaf springs designed and intended to be used like you want I'm not sure if Mopar still offers a set for a 1964 to 1970 B body 4 speed car for all around high performance drinving like you want to do I bought a set of the Mopar S.S. road race type leaf springs for my 1966 Coronet, they are the shorter front half length like the A bodys are so the front spring hangars would need to be replaced to keep the rear end in the stock location if you want to do that. I didn't use them as I moved the leaf springs into the rear sub frames and moved the rear end forward two inches, which required reshaping the front fender openings
I'm sure your leaf springes are worn out so replacing them will help the car a bunch, if you want to try a new set first before adding clamps do that If you want to add the clamps later I use 1/4 thick by 1.0 inch wide flat stock steel and drill 3/8 holes in them on each side and use 3/8 bolts of the proper length for each clamp, I place them on the front half of the spring in every where I can (the tire bulge will, may, hinder using one on each side near the tire bulge ) on the front of each shorter leaf springs to clamp it to the main leaf. Do it with the weight of the car on them, don't jack it up on jack stands with the rear end hanging down You can jack it up and put jack stands under the rear end axle tubes and under the front suspension so it has the weight on the suspension to do that to be able to get under it to work on the car
Let us know what you do and the results