The 73-76 A-bodies differ from the earlier models because of federal legislation regarding impact standards, the entire suspension sysetem and emission standards changed about 1973. Many body parts will interchange between years but the change you are contemplating will probably be labor intensive and very costly. Unless you have an abundance of time and money this project will not be a satisfactory endeavor.
if your taking everything from the other front end it should be a bolt on. The only difference on the fenders is a mounting tab (but wont matter if you do the entire swap) and a different marker light. 70-71 had the inlaid marker light and later had one that wasn't inlaid.