speedometer not working

[chev-obsession]

So about a year ago I finally got my 65 running and driving. I swapped in a 700R4 and used a calculator online to find out which speedo gears I need for the trans. cant remember what gears they are but when I drove it, it was off by about 50 mph.

After about 30 miles of driving the speedometer finally quit working all of a sudden and hasn't moved since. So since the cable is new and it was reading 50 mph off, some thing tells me that's not the problem.

So my 3 questions are:

1) Since the calculator that I used was obviously wrong, what speedometer gears for the transmission do I need for the speed to be right?

2) what likely happened to the speedo in the gauge cluster

3) how do I remove the gauge cluster? I have tried taking it out before but I could not get the light switch out.