Hi there hj308,

I drive a 1959 Oldsmobile left hand drive, somewhat larger than a Mustang (18 1/2 foot long), Took all of about a day to get used to it. Now I think nothing of driving on the wrong side of the car.
Enough Mustangs have been converted so should not cost too much to change over, depends if you want originality and that bit of 'wow' factor when people see it's original.