$300-$500 range, for a completely custom built all aluminum MT? How in the world could you ever do that and still make money?

I'm assuming this truck is based on off the shelf diffs and transmission? Out of what truck? You might be better off offering it as an upgrade package for whatever type truck the diffs came from. That's the way it will be viewed anyway if it uses existing drivetrain assemblies. I can build a pan chassis and totally reconfigure a Savage to where the only thing left stock on it is the diff design... but people would still refer to it as a custom Savage.