You don't say what type of ignition system is in the car. If it is a point ignition system then the coil measurement of 5 or 6 volts would be normal. Points systems use a resistor, could be a block style or a resistance wire and some GM's used an internal resistor coil. For testing purposes, make an install a jumper from the battery positive to the coil positive, test run for a short time, NOTE - this many volts can and will eventually cause harm to the points / condensor / coil.. but a short run to help diagnose will be fine. Also, again assuming it's a points system, have you cleaned the point contacts? The 2 faces routinely build up a layer of corrosion which can be removed pretty easily with a points file or even a piece of newspaper placed between the contacts and pulled through. I wouldn't replace the coil yet. The ignition system has many parts and replacing them piece by piece gets boring, time consuming and expensive. Please confirm what type of system you have and if it is a points system, then the voltage drop is normal. (more than likely)