I ordered a Distributor Advanced Curve Kit that was for GM HEI distributors... it was $9 and the weights and springs looked like mine in the picture (which I admit is not a very scientific way to choose replacement parts). I received it today and all components looked very similar to the parts in my distributor. I went ahead and replaced my advance springs with the most-similar set that came with the kit.
After putting the distributor back together I fired it up, checked that the initial timing was still good and then measured the advance at 500 rpm increments up through 4500 rpm. With the new springs the timing advanced a little more slowly than specification but flatted out right on spec at about 28 degrees. It had previously climbed past 45 degrees at 4500 rpm.
I'm going to keep my fingers crossed and see how it does on the water this weekend. I'm optimistic!
Thanks all for your thoughtful feedback. I'll post an update as soon as possible.