By "regular" stock do you mean a fixed a2 rifle style stock? If that's the case you'll need a new buffer tube, spring and buffer. Best to just buy a complete stock kit that includes everything.
This kit shows everything that you need. http://palmettostatearmory.com/a2-rifle-stock-kit.html
I think bushmaster may have used locktite on their buffer tubes at some point, so watch out for that, otherwise just get your castle nut backed off, unscrew the tube, watching out for the take down pin detent spring and the buffer retainer. Screw in the the rifle tube and put on the rifle stock with appropriate spacer and screw and install the rifle buffer and spring.
If you mean you want to install a mil spec telescoping stock, like m4 stock, then all you need is the mil spec tube and stock, all other parts are the same.