[Captainfab]

Here are some pics of my '66 C30. It's a longer wheelbase than yours, but the configuration of the frame would be the same. And as you can see the leaf springs in the rear. GM did use coil springs in the rear of both C10 and C20 trucks from 1960 thru 1972. However leafs were available as an option, but a little more common on GMC's. Like I said before, all the C30's I've seen in these years were leaf springs in the rear.