Scale length is the total length from the nut to the bridge. It’s not the length of the neck. A 37” truss rod would be longer than the neck itself. Where would you put it?
Your guitar was built to a specific scale length. You don’t want to mess with that.
The truss rod can only act on the part of the neck that’s between the neck joint and the nut, so that’s how long it should be.
But I’m confused — you say you’re building a neck. You’ll have to rout that neck to accept the truss rod. If it’s a preexisting neck, it’s already routed for a truss rod, and there’s already one installed. If you’re building the neck from scratch, take a moment to gather your thoughts. You’re asking questions that indicate that maybe building a neck from scratch is a bit too ambitious at this stage.
