My experience with Big Twins is limited to five motors including three 1956 Johnson 30HP. But I suggest that you just concentrate on getting the motor to run. First address the lower crank seal. You can change shaft length later if you care and address any safety concerns you have once it runs. In my limited experience to correct the lower seal issue you will wind up opening the crankcase anyway. If you do open the crankcase, you should replace the top crank seal too. (easy).
As discussed, to change the seal from lip seal to carbon type you would change the lower bearing, and add the parts associated with the carbon seal.
The carbon seal parts are shown in the parts list for the 1956 Big Twins.
The 1955/56 BigTwins are sweet running motors worth the trouble and cost to bring back into service.
