To close the loop on this: Figured it was time to stop thinking about it and actually do it, since I just completed a 12-day road trip carrying a lot of stuff and pulling a boat, during which the RR wheel well was within a cm of the tire - turns out that in addition to being old and soft, a little less than a turn had broken off of the bottom of the spring (it was intact when I replaced the shocks last summer). When I called the dealer to place an order, the parts guy asked for the last eight, and I don't have a Baja, so I called my buddy who does and he supplied his... and the parts guy said that it listed the 20380AE39A spring, which only confused things. So I ordered two of the 50A, and the idiots in the (Portland?) warehouse shipped one, and I put it in yesterday and it fits fine and looks great, though I won't really know until I load it up. But if it's stiffer, I should be made in the shade.