So after watching the video a whole bunch of times, I'm still not entirely sure what's going on in that first tire revolution. It looks to me like there was a tiny bit of tire slip. Now, whether the tires slipped because of the tires, or the tires slipped because of a spring reaction is impossible to tell from that video. In the first revolution the springs are unloading, the video clearly shows the suspension loading as you prepare to launch (as it should). That little hiccup as you launch and the springs release could be darn near anything. A small bind in the release of the springs would do that, and based on what's in the video it's really small. Could be a tight spot in the shackles, a bind in the leafs themselves as the slide a little bit over each other, a shock issue, you name it. If you are running shackles I would check to make sure they're not overtightened and that they're properly lubed. Same for the U-bolts and everything else, if something is overtightened it can cause binding. Make sure everything is torqued to the proper spec.
The SS springs are sold by vehicle weight. The only exception is the 002/003 springs, which are the original pro-stock springs and didn't have their part #'s changed in 1974 when Mopar reclassified all the SS springs and gave them new part #'s. But they also have an associated vehicle weight. You can see the currently available springs at Summit Racing Springs - mopar-performance-springs
This page is out of a "Mopar rear suspension racing manual", released January 1980 as PN P4007927, Bulletin #30. It covers a bunch of different suspensions, but has a lot of info on the SS spring set ups. Unfortunately even just the leaf spring section is too big to attach, even as a zip file. It lists most of the available SS springs now, looks like a couple may have been added since then.
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Like Rob said I seem to recall there being more to it than just picking your vehicle weight and buying the corresponding spring. I think there were some corrections for that and I was hoping they were in the bulletin but I didn't see them right off and I'm not gonna go back through all 68 pages right now. I do know that when I ran 3400# SS springs on my Challenger I found them to be ridiculously stiff, but I was running it on the street. Similarly I know that a lot of the guys that run the 002/003 springs have cars that probably weigh a bit more than 3,000 lbs.
But I will readily admit I'm not a drag racing guy. Drag racing spring rates tend to be a lot higher than what you'd want to run on the street. For street and handling performance usually an A-body is good with 120-130 lb/in leafs. But for drag racing the 002/003 springs are usually the go to, and they're 160 lb/in. Obviously it all can change with car weight and set up, those are just general numbers.
