If you haven't yet bought any parts and were waiting to see what it was like inside then I'd seriously consider getting a box from a low km car from a wreckers rather than continue with what you have. When I was looking at replacing the weak overdrive in my GU and doing a box rebuild at the same time while replacing the clutch, the prices I got were something like $700 for bearings, seals and syncro springs, $350 for overdrive gear and main shaft, and to top it all off, $150 per synchro ring with the box containing 6 of them. I priced a second hand box from a wreckers and it was under $2k while a professionally reconditioned exchange box with warranty was under $3k.
I ended up doing nothing and putting it back in how it was since the box was working fine at the time. Still waiting for 5th gear to blow up.