Hi David, welcome to the Forum and good luck with building your CK. You will have a lot of fun I assure you. The Place a few of us have used for leaf springs is this https://www.midlandmotorsprings.com/specialist-springs
My own springs are semi elliptic, 500mm between eyes and 60mm arch. My Rear springs are stiffer than the front but I can't remember the sizes. I think Fronts are 38mm x 4.7 and the rears 38mm x 6.35