Using your leaf springs would be the simplest and cheapest way, would also give you a decent ride. Leaf springs also leave you room to route exhaust. Also given your body style, the rear suspension would not be real visible, so spending the extra to install a 4 bar or 4 link would not pay off with a lot of visual impact to most folks viewing the car. You would need to locate and install anchor points on the frame for your springs. On the other hand leaf spring suspensions are not nearly as trick as the others and are not as easily adjusted for ride height or tuned for performance. Good luck with your project

Pat