Why was the transmission pulled in the first place? Maybe the rebuilder re-installed a part incorrectly?
If you are 100% sure it is the governor, most of the time it is just a grounding issue. Check and re-check the wiring. As I was going through my BW R10 overdrive in my 55, I too had issues and it was a simple wiring issue where I had not installed the correct terminal back to the kickdown relay. I know that is not the governor as you indicate in this instance, but once I double-checked my wiring we were good to go.
A simple test on the governor would be to hook up a test light in the car clamped to a known LIVE terminal. Then the other clip to a lead wire going to the wire/terminal of the governor. Once you are up to the magic speed of 28 mph, that governor should ground the lead wire and voila! the light comes on.
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