I replaced my original speedo with an electronic Smiths speedo. I ended up driving it with a GPS antenna mounted in my engine compartment (along with some other electronics). It works great but there is a downside. It takes a couple of minutes to acquire enough satellites after I leave my garage before it can register my speed (and mileage) and when I start under tree cover, it can't acquire the satellites till I get into a clearing for a long enough period of time to acquire them. There is some logic in the antenna that remembers (for a limited time) where the last satellites were so when I park, it doesn't take any time to acquire them so it's really only a problem when I first drive away.
Over all, I'm really happy with it. As you've found out, there are many ways to skin this cat. The electronic driver driven by the speedo gear in the tranny is maybe a better alternative but I shredded my speedo gear so that wasn't an option.