So why not make the boltin plugs for the gears if the spline is the same?

On the Viscous diff I would stay away from it for racing unless you want to spend more money on it than a clutch type. They don't last long in a race car. The options for finding the correct outputs for your 280z are none as far as I remember. you may be able to have one custom made but that would be pricey. It would be easier to convert your car over to CV's and build custom length CV axels for it but I assume this is an IT car?