If your WB was originally a 6cyl it probably has a 3.55 diff in it, possibly 3.36 if it was an auto. Putting a 3.08 will make it more sluggish. You won't find a small salisbury diff lower than 3.55. If you want to run 3.9:1 you can easily do that by finding a 6cyl HQ-HZ ute or van diff housing and coarse spline and fit that to your WB. Grab a LH-LX Sunbird or 4cyl Torana 3.9:1 diff centre for it and you are away. Tailshaft, brakes, U-bolts etc are all identical. Your best bet would be a HZ housing though as it'll have the RTS sway bar mounts on it. You can also fit a V8 diff into it (big 10-bolt) from a HQ-WB ute, can or tonner but these come in the same ratios, only one lower than the 3.55 is a 4.44. This too will fit straight in but will need tailshaft mods.

If it has the factory V6 memcal in it with a 3.55 or 3.08 diff the lock up torque converter and speed sensor (and cruise control) won't be working correctly. You need to get a memcal re-written with the right ppk (pulse per kM) constant for the tyre diameter and diff ratio. Things like idle when coating to lights etc can be affected.