I'm not an expert by any measure, but my I'd look for a poor connection, broken wire, bad ground, etc. first. As fast as that needle's moving, my best guess is an electrical problem.
Incorrect timing or plugged exhaust can cause overheating also. Had an old Dodge big block truck years ago, don't recall what I was doing to it but I severely retarded the timing. Ran smooth, but was gutless as all getout and exhaust manifolds were cherry red.
Had another one that would overheat intermittently, assumed it was a cooling system problem, went over that part several times but found no issues. Finally discovered that the flapper valve in the driver's side exhaust manifold(the one with the thermostatic clockspring just above Y-pipe connection)was occasionally sticking shut.