If it is the stock ignition system then there is supposed to be a resistor in the ignition circuit. This will screw with your tach readings because 12V is not being seen at the coil. I ran a new ignition wire and bought a HEI distributor, better ignition system and solved the tach problem.
I ran into the constant starter problem the other day, trying to bump the engine with a jumper wire for setting the timing since my crank bolt is inaccessible. Thank goodness for that disconnect I installed. I am having problems with the starter too, it is acting like it has an inconsistent ground, but its grounded enough to smoke a jumper wire in a few seconds. I'll probably go with that motor listed above, as I'm pretty sure this is the original starter. My solenoid is almost new, and you can hear it opening and closing so the problem is def the starter.
Try tracing the wiring for the wiper motor, you might have a bad ground or the speed switch has a short in it that is triggered by the engine running. Does it have that issue with the ignition on, or only when the engine is actually running?