PatB,
One of the more obvious tweaks that must be done for a lefty is to properly configure the extractor and ejector such that the hot brass in not forcibly embedded in ones face...
That reality check having been resolved, there is no other practical limitation.
The "manual of arms" for the Garand was designed before southpaws were given social recognition for anything other than a mean fastball. For informal plinking, hunting and target shooting, what ever safely accomplishes the function would be acceptable. For drill teams, however, someone is gonna have to learn to do some "other-handed" stuff...