I would suggest and recommend "T" Springs in W Hartford Ct. Talk with Travis, he is great to deal with and makes a quality spring. Also a connoisseur of fine micro-beers
I installed new front and rears on my 26 Fordor. He would be able to build or recommend a proper spring for your sprint car.
www.T-SPRINGS.com or 1-860-233-3557 Good luck and let us know how you make out.