Kara, you have a multi-leaf rear with multi leaf springs, but monoleaf shock plates and presumably shocks intended for a monoleaf suspension.
This could very well be the problem. The mono plates shock mounts are lower than the multi plates, about an inch, I believe the shocks are different lengths, too.
Regardless, if you unbolt the bottom of the shock and fully extend it, with the car on the ground and the weight on the wheels, the shock eye should be a few inches below the moutning hole in the perch, allowing the suspension go down from it's static position when you go over bumps.
How is the ride height in the back, good or too high?
If you tightened the springs with the car in the air, you might loosen those bolts and bounce the car a bit, it may settle down some.
You may need different shock, longer ones, if the ride ht is ok.
I don't think anyone reproduces 67 multi leaf shock plates, but some have "re-arranged" 68-69 plates with a saw and a welder.
btw, welcome to Team Camaro!