I bought a set of '99 talls used, and found that the rears had already sagged once I installed them, so I put a 10mm spacer on the rear, and it gave me about a 1/2-3/4" lift over my '96 stock springs I replaced. It had an even stance like this until you put anything in the back or hooked up a trailer, then the rear sagged again. I kept the '99 tall fronts, and swapped the rears to the OME 906 spings, and gained about an inch in the back giving me a bit of a rake (not much, looked really good), and never had squat problems again, far less body roll, much better ride, and predictible handling as a result.
906's in the rear are all win IMO. I'd have no problems buying used '99 talls, but I would only plan on using the fronts, skip the '99 rears altogether as they don't have enough springrate and sag too easily.