By "5-9 stock", what red means is the siize that was used from '55-'59 as an oe wheel.
As to what width you'll need, that will depend on what size tire you want to run. A wider tire will need a wider rim for proper sidewall support. Also, tire construction type comes into play, are you wanting original type bias, or modern radials? That will determine what sizes are available to you. I'm going to go out on a limb and take it you're considering radials, since feul efficiency is a concern (radials help with that, lower rolling resistance). With a 16" rim, and looking for a tall skinny tire, the choices will be a 215/85/16 or a 235/85/16. You can get away with a 6" rim with the 215, the 235 really works better on a 7" rim. Bear in mind, both of those sizes will be E load range (10 ply) and will make for a harsher ride quality.
