Putting a bit of load in the back is just trying to change the harmonic at which the bouncing occurs. Putting weight in the rear also compresses the springs providing a little bit more 'pre-load'. Just trying to find out whether the 180B has the same problem as the 1600 in that the rear end flaps around when you get a bit of axle tramp going. This only makes it worse as the rear (from the shock towers) isn't really that well supported. Hence the reason I put some sheet metal diagonals to strengthen up the rear.
It's a quick way of finding out if it's structural or not.
"Rebound" is the rate at which the shock extends back to normal over a certain extension speed/load/time. By having this adjustable, you can tune it to the road surface, or the spring you are running (or both). You want the system to be close or actually critically damped (do a search on google). Currently your system may be under damped, which means you need to adjust the rebound to suit the spring/system. Hence the adjustment. The range I am referring to is typically for a heavier car (the holden commodore) which usually runs heavier springs (like the 240K KDRL07 for example), so you need a rebound setting similar to this. The ranges that I provided (both the single adjustable and doubles) should provide you enough play to work with both spring rates you referred to. It's just a case of a few clicks on the top adjuster for rebound.
Double adjustable is basically a rebound and bump (extension and compression) adjustable shock absorber. It allows you to change both bump and rebound usually independently of each other. The 8242 has both. 3 times the price puts it at around $1300 ish for the pair. Well worth it IMHO.
My idea would be to borrow a set of shocks that are single/double adjustable from someone, put them in real soft on rebound (worry about bump/compression later), watch and feel the rear end bounce. Go back, put them on the tightest setting which is hopefully when you watch/feel as you wheel spin. By finding the sweet spot (depending on the spring) between these, you will have found nirvana. In some instances, you may want a little wheel spin (doesn't bog the motor down), so you adjust the shocks a little stiffer than the sweet spot. Having the shock too stiff means you don't get the car to squat (weight transfer to the rear), which usually means wheelspin!
If you can't get the car to stop the bounce, then you need a stiffer rate which means the adjustment of the hopefully borrowed shocks isn't enough and you can return them and try something harder...
The other thing that no one has mentioned on here is tweaking the front end too. The front end typically tries to come off the ground when you dump the clutch. If the spring rate/shock rate on the front isn't 'matched' to the rear, you will get a 'resonance/harmonic' and the front and rear fight against each other kind of giving you the bounce you <may> be referring to.
I've probably forgotten heaps, and worded everything really crappy, but that's what you get typing this stuff out on a phone screen
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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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