Clark: I would make sure of two things:
1. Check for continuity (or zero volts) between the starter case and the negative terminal of the battery, in other words, make sure the starter is well grounded.
2. Check to be sure that the solenoid is passing the high current on the load contacts. That is, if the solenoid is clicking, it means that the safety switch, key, and solenoid coil are all working; what you need to make sure of is that the battery power on the big post is being passed from the terminal with three wires on it to the terminal with one big red wire on it.
Bottom line: If you have 12 volts coming into and out of the solenoid with the key in the start position, you should have 12 volts at the "A" terminal of the starter/generator and the sucker should be turning over. Again, if you have 12 volts at the starter and it IS NOT turning over, then either the engine is locked up or there is a problem with the starter. (If the engine is locked up, a voltage drop at the battery [or heat at the starter] would indicate excessive current draw.)
The schematic for reference:
Edit: Also make sure the battery is well grounded; your ground could be able to support the low current demands of the solenoid coil, but balk at passing the high current demanded by the starter.
