To answer some of your questions directly... The initial mechanical setting of the float level is just to get into the ballpark. The sight glass is the real test. The desired level does depend on the carb design though. What year are you working on? 73 or 74? Can you post a pic or two of the power valves? That's another easy way to know for sure. However, without even seeing the carbs, I suspect the fuel level should be right at the dot in the middle of the window and your level is way too high.
Tips for cracking the power valve open? Not really. The gaskets hold tight and can make it difficult to separate the wafer stack. Other than brute force, I don't have any suggestions.
And testing the carb off the car? I don't have any way to do that.
If it was running great one day, and then the next day it wouldn't start at all, I'm not convinced that it was a carb problem in the beginning. But now that you've opened them up and let the magic out, there might be one now.
Did you try starting it by holding your foot down a little on the pedal while you cranked it?