Hy Iron Clad, I don't think the NV4500 ever had a driveshaft parking brake, so that could be a problem to fabricate. there are older Clark, Spicer, or New Process with overdrive which would be more of a drop in, but they will be more difficult to locate, and probably more expensive. You could change the whole rear axle to a two speed Eaton, or find a lower ratio for that housing. Getting taller tires will help as well. There really are a lot of choices when you want to increase speed, you need to decide what your level of expertise is and what you can do or afford to pay someone else to do for you.
