Adding a valve slice is easy. Adding the linkage on a cab tractor can be difficult, open station tractors are easy. Kits have decent instructions. Paying for the kit hurts.
A cab is harder because the lever pivot supports more then one lever and is bolted to the underside of the cab and covered inside by trim panels. The levers on non-cab models are much more spartan and usually can be assembled on the fender while sitting in the seat.