I recently tried to make some late Stug IVs using Plastic Soldier Company's Stug III and Panzer IV kits. I really wasn't happy with the results as the proportions are all wrong.
I tried again with the Flames of War kits and the results are far better and a much easier conversion to do. The modded exhausts and driver's hatch are all from parts off the sprue and the sprue frame. Only needed some plasticard to make the top hull plate. I also found the Flames of War plastic is much better to work with because it is far less brittle than the PSC plastic.