Heres a link to a company that offers an upgrade for the W/C version: http://www.5speeds.com/t5/gforce.html
There are a couple differences between the W/C and non-W/C.
1. Mainshaft has a bearingless design for 1st, 2nd, and 3rd
2. Countershaft uses flat roller bearings
3. Uses 1 piece brass blocker rings throughout
1. Mainshaft uses bearings for 1st, 2nd, and 3rd
2. Countershaft uses tapered roller bearings
3. Uses 3 piece blocker rings on 1st and 2nd, and fiber type
rings on 3rd and 4th for longer life
A regular rebuild kit for a W/C won't work on your non-W/C.
As far as converting your non-W/C to a W/C by changing all of the gears, bearings, and mainshaft from an upgrade kit such as the link I posted, I couldn't say for sure. I believe the cases are slightly different for the 2 types of countershaft bearings.