I just did this conversion myself, using Beastmen parts on my Khorne Berserkers. If you have a jeweler's saw or a small hacksaw you can cut them directly in half, just keep in mind that the groove made by the saw will remove mass from the model so place it either above or below the line you want to cut at. The above method worked the best, though. Since the waist has a belt/buckle and is uneven on some of them I just used my x-acto knife to cut down into the waist where I wanted to separate it and then snapped it in half - it broke along the premade cut.