Voltage on the - side of the coil with the points closed SHOULD BE ZERO VOLTS. That is where your problem is. Either the points are defective, the wire from the coil - to the points is defective, the distributor is not "grounded" (not a separate wire, just the physical contact to the engine block) or maybe (I don't know if these distributors has one, but some do) the wire under the plate in the distributor that connects to the case of the distributor is missing/broken.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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