need info on the rearend in my truck

hi, i have 1981 chevy scotsdale. I wondering what size ring gear is in my rear end. Its a ten bolt and the gear ratio is 250 (i think, i took the cover off mounths ago and i didn't rite it down). I want to change the gear ratio and i would also like to convert it to a limmited slip posi. So the question is what size is the ring gear and can i make it into a posi and how much would this cost.

Unless its been changed ,the book lists 8.5 ring gear in a 10 bolt from starting in 80 .....here is a link ......
Automotive Parts Guide for GM 8.5 inch Differentials, Ring & Pinions - Save Money

is there a cheaper option to improving my rearend????

a 2:50 ratio?? usually 6 poppers had steeper gears then that.