With that info, have you tried finding a threaded plug or bolt to install in the hole to plug it up? I'd measure the hole and thread count/pitch and go to your local hardware store to find a pipe plug you can thread in there with some sealant.
There isn't a correct original or after market gear set to use with your 3.55 rear gear ratio, although you could use a speedometer ratio adapter. It would have to be geared to correct the original output speed and correspond with what your speedometer cable input speed would need to be for accurate speed readings.
Best of luck sir!
