Bridge it baby

Hey all, i'm trying to track down another diff head for my rx7. Its a series 3 with the larger 26 spline axles and larger diff housing than the series 1 and 2. I've managed to find a mazda bongo van that fits in but its only a 4.1-1 ratio, i'm really after at least 4.5, but ideally a 4.8 or a 5.1-1. I'm starting to race at the drags and the standard 3.9 is waaaay to tall for any decent times. I'm getting some 205/50/15 tarmac rally tyres (super sticky) as well, so hopefully having these two things done will give a better result than a 16.01 ET I'm not even redlining 3rd gear with that 3.9 diff.
If anyone has any suggestions on wat diff i can use, much appreicated!!