Re: rewiring 65 fastback
Do all gauges peg out? Typically that's a voltage regulator problem, but you're already tried that. (Dumb question, but when you say regulator you're talking about the tiny silver one behind the instrument cluster, right? Not the larger one near the battery in the engine bay?)
The fuel gauge could spike if the wire was grounding somewhere along the line back to the sending unit in the tank. Maybe the other gauges are having the same issue.