Hello Luke,
Correct grill for your '58 car would have been the chrome on brass item. IMO it looks far more handsome than the (admittedly lighter) aluminium version that came along somewhere in 1959. The chrome on brass grills are expensive to restore, but (again IMO!) worth it in the end.
Should your car have a chrome on brass grill, and if it will be surplus to requirements after you have secured an ali grill, then I'd be keen to buy it please! Tim