If your 53 has the original engine, its a low pressure version with babbit bearings. I wouldn't want to wind it up much past 45 mph very long for the sake of the rod bearings. My 51 4400 has a later high pressure 235 and the same gears as yours and its really wound up at 50.
About your bouncy speedo, you might try lubing the cable. I like to use graphite, the kind they sell for lubing door locks at the auto parts place. Pull the speedo cable off and chuck one end in a cordless drill and lube from one end until you see some on the cable at the other. If it looks really gummy, it might help to soak the cable solvent first (blow the solvent out before lubing).
