Your ZP is a posi rear end that originally came with a 3.90 gear ratio (39/10) gears. I just went through a gear change myself and found that it is near impossible to find a gear set you can use in your carrier. They splines you need are BOP specific and most pinion gears do not have the 27 spline. If you change carrier and get a complete set of carrier and matching gears can be done, but if you're particular about retaining quality and originality you may decide against it. Plus if you research carrier manufacturers you soon find out they have noisy and unreliable units. Best bet is to talk to a reputable gear shop, in the area and pick their brain. I ended up to leaving the carrier stock and added a shim to be able to use a higher gear set using a stock gear set. Worked out great, you can do it yourself, take your time, get a mag base indicator set, shim stock and slowly do it in 2 weekends. I have 2 OEM sets out of my Firebirds up for sale. One is a 2.78 the other 3.55 set if you're interested.