Rear suspension porposing on hard acceleration in 1st & 2nd

On anything but a very smooth road and under full throttle in 1st & 2nd gear, the rear of my car lifts up to plant the tires. But, then it feels like the rear of the car is jumping of the ground, spinning, landing and then doing it again and again.

Once it's in 3rd gear, it stops and feels like its planting the tires hard. It's a manual transmission car with SS springs and Bilstein Shocks. I've tried QA1 double adjustables, but they didn't seem to be properly valved for the range of adjustment I need. Bilstein makes 3 valving options for the longer shocks I choose and I went with their middle valving.

I suspect I need more compression and rebound dampening on my rear shocks, but I'm also trying to avoid a harsh ride on the street. It seems like I need to slow down the extension or rebound of the shocks.

Has anyone corrected this issue, and if so how?

Is the back of the car sitting up high.. but you need to tighten the extension on the rears

You hit it on the head. Shocks aren't controlling the rear housing. Couple this with the fact that with stock rear shackle location the spring may be stretched full out. The rear shock may be to short topping out. The rear shackle likely needs to be moved forward or replaced with a slider(also forward). get the correct length shock. Talk to the supplier about valving options. I'd bet a stick car needs stiff.
Doug

I don't think the rear is sitting too high, but here's a picture of it.

"You hit it on the head. Shocks aren't controlling the rear housing. Couple this with the fact that with stock rear shackle location the spring may be stretched full out. The rear shock may be to short topping out. The rear shackle likely needs to be moved forward or replaced with a slider(also forward). get the correct length shock. Talk to the supplier about valving options. I'd bet a stick car needs stiff.
Doug"

Thanks you guys. I'll disconnect the shocks, let the rear end hang down fully and verify my shocks are not restricting the extension. I'll try and get a picture of my rear shackle position, and look into installing sliders to replace the stock shackles.

Looks to have caltracs???? If so looks like bars are in upper hole. Putting in lower hole softens the hit and wont make it so violent . then play with shocks.

Back in around 1981 my 340 Cuda, stock suspension, snubber, raised car 1" with longer shackles to clear slicks, std comp shocks with 2" shock extensions....angle of rear shackle in pic....moved near 4" forward on the hit.
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Does it have cal tracs and SS springs?

If so.......Try taking the bars off.

I'm actually running 3200 lbs SS springs with a properly adjusted slapper bar. Without the slapper bars, the car was bending the front passenger side spring segment and a pinion snubber basically stops any movement of the rear suspension on the open road resulting in a brutally harsh ride. The spring rating of 3200 lbs is lighter than the car by at least 500 to 700 lbs for better ride quality.

You either need the slapped bars OR the SS springs. Not both.

I would sell the SS springs to someone. Anyone. They are all but useless on a stick car. And marginal on an automatic car.

Now everyone can post how quick and fast their cars are running them. I don't care.

Check the rear shackle angle, it needs to be angled further back towards the rear bumper at full lift, not leaning forward at all
Good luck
I raced a 1970 Baracuda Hemi 4 seed car for a little over 2 1/2 yrs in NHRA stock class, it had issues (worn clutch plates) in the stock Dana making it pull to the right, no bouncing or any other traction issues on the starting line, I rebuilt it and that stop that
I put some slapper bars on it before fixing the posi unit and later removed them for spring clamps on the front half of both springs
It was a real tire spinner on nine inch slicks though
I swapped flywheels form the stock 40 lb.170 tooth down to a 33 lb. 143 tooth and again down to a 30 lb.130 tooth and finally to a 18 Lb. aluminum 130 tooth which made the car bog on the starting line with a 2.65 low gear ratio and a 5.13 ratio in the Dana I had swap the rear gears also from 4.10 to 4.89 and finally to the 5.13 to make it run 7000 RPM in the lights in the 1/4 mile
That car wore me out testing, I sold it 2 1/2 years later after finally weighing it on certified scales at the old Irwindale drag strip, it had 2160 Lbs. on the front tires and 1280 Lbs. on the rears without me in it I took it home after that and never race it again

The housing just hanging may not be a true indicator of how far the axle moves away from the body. Take some small string and tape it securely to the top/stationary side of the shock. Pull the string down and tape it lightly to the lower body of the shock and mark the string with a sharpy at the edge of the tape. Make a run, it will pull the string out from under the tape. Mark it with the sharpy, now you know how much extension you are really getting. Disconnect the shock and extend it until the string has no slack. Is there more shock travel?

First thing is to check the U-bolts and make sure they haven't come loose. trq them to proper spec. Then make sure the purches are sitting perfectly flat on the leaf.

I had the same issues as you have described. What's happening is that your not controlling the rear end, in other words you're getting too much separation and either your shocks are running out of travel or your leaf springs are stretched out to the max.
I was running split mono leafs, cal tracs and off the shelf QA1 double adjustables which were not valved properly to control things. With rebound tightened to the max the rear was still slamming out. What did help the situation was to add rebound to the front double adjustables. That slowed the front end rise, which settled the rear some. But on launch I would pull the front wheels, come down and the rear tires would lift off, come down and pull the front wheels again. It was ridiculous. And from all that the spring searches curled, and there went the pinion angle. Hope your perches are boxed, if they flattened that diff will be rocking and rolling under there.
Got to hold the rear diff from slamming out.