Otto, like Chip said, only two bulbs in the gauge cluster, and only 2 illumination bulbs in the speedo, plus the one for high beam indicator, which is on a different circuit. So the 4 illumination bulbs for the cluster and speedo, plus the one bulb for the ignition switch are all that should be on the harness that you're going to connect into the taillight terminal. I doubt that the difference in wattage between the 55's and the 51's will overload the switch capacity.
Even with the 51's, the white paint inside the gauge cluster got toasted a bit over the 70 or so years they were in there. The 55's might darken the paint a bit more than the 51's if that's a concern.
