Things to B done

I just cracked open my tranny on my 86 just to see if anything is worn in it while I have it down. I have a couple of gears from the reverse Idler gear missing. Its a 16V tranny. Anyone have a used one for sale in good condition? I am also looking for a lower fifht gear conversion at the same time. I am also curious about the limited slip diff kits that I see for sale. Do they actually help? Would this be a good time to put one in or because I have my tranny all apart is there something better I could do? I just didn't think for a mear $70 that the kit could do that much? What gear would you go with as a final for fifth if you had a choice?