67 - Rear coil spring question

[oldman3]

I redid a friends 67 pickup a couple years ago. We installed a drop kit from CPP. We did a 5" coil spring drop in the rear, with relocators for shocks, c-notch, etc. The pickup is a 67 longbed stepside. It was orginally a 1/2 ton heavy duty long bed. We kept the long bed and added a gas tank at rear of frame. What is happening now is when you ran down the freeway at about 70 mph, the rear axle bounces like a rubber ball. It's not real bad, but you can see it if you watch from another vehicle. I'v talked with CPP and they think I should go to the 4" heavy duty drop coil spring. I think this may take care of the problem, but wanted to check with people on here who may have had this problem, or can help out, before we order the new springs. I have attached several pics, showing old springs & new, and the current underside of pickup. Let me know what you think.

Pic 1 old and new springs
Pic 2-5 shots of undercarriage