I recently found a booklet on flame weapons issued 2 June 1945 that shed some light on the Flamethrowing Pershing and why it had so many options considered -- internal gun, external pack; trailer, etc.
Quote:
No Satisfactory Method To Mount in Heavy Tank
Mounting a flamethrower in the T26 and other heavy tank models presents apparently insuperable obstacles, at least at this time. The heavy armor and crowded condition of the fighting compartment apparently bars installation of a flamethrower fuel and compression system large enough to be effective. Research is continuing.
Pumped - Fuel Flamethrower Considered
Standardization of Napalm, which can be pumped successfully, has turned some attention back to the pumped-fuel idea. Use of pumps, rather than pressure vessels, to propel the fuel will mean a saving in space, since the pump can be designed to fit more crowded and irregularly-shaped than spherical pressure vessels. Research is under way on this.
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So it appears to me (speculation) that all US flame tanks used pressurized gasses such as nitrogen or carbon dioxide to propel napalm/gasoline out of the flame gun nozzles.
Providing the pressurant needed spherical pressurizant tanks; which could be accommodated in the Sherman easily, thanks to the Sherman having sponsons to shove things into to get them out of the way so that the spheres could be accomodated. The more modern Pershing and the other heavy tanks (T29/T30) were built around "space engineering" concepts which abolished the sponsons; so putting flamethrowers in them with a useable amount of fuel required development of a pump-fed system.