There is no difference in the grille.

Keep your same front end and just get a new (wider) rear bumper and put the rubber filler between it and the body (all 74's had this). You'll also have to change the rear bumper mounting system from brackets to shocks (this gives the gap which is filled by the rubber).

Presto its a '74 (if they don't check your VIN).

You can lead a gift horse to water, but you
can't look in his mouth.