When you say 'negative' rake, you mean higher in front? That's no bueno...
My understanding, and take this with a grain of salt, because I do not have the 880s or 881, I've only installed the 906 and 5100 in the rear. The 906 gives you around 2 inches of lift. Mine was sagging a bit so I got a bit more. Supposedly it settles to around 1.75 inches. My stance right now is great, and I have around a 1" rake from back to front. Mine is a 98, but it sits higher than the 2000s and 2001s I've seen, so I think my front is taller to begin with.
Supposedly the 880 is around 1.5 inches of lift, probably a little less once settled also, so - the 906 880 combo gives you a slight lift with about a .5" rake - front to back. The 881 is the same weight coil - so it's made in the same way as the 880, just slightly longer, giving around 2-2.5 inches of lift.
So at BEST, the 881 gives you a level stance - and possibly a slightly higher front than back. The 880 gives you a noticeable rake.
The 883 is actually made for more weight in the front, and still providing that lift.
So, at least for me, I'm going to go with the 880s in the front, and leave that slight rake.