This has been talked many times. Just making a search will find the answers. Basically:
-You need to find all the Hideaways setup... Grilles, doors, motor, doors rod and all related brackets.
-Will need to remove the bottom bumper reinforcement to be able to fit all the brackets straight to the bumper ( 73/74 bumper beam is right in the middle of these provisions ).
-For safety, is better or nice also get a beam reinforcement which attachs on inner brakets of the Y bumper brackets of the 71/72 ( not used on 73/74 ). This will require drill some holes on Y brackets too. NOT MANDATORY FOR THE ASSEMBLY but better for safety in a crash incident.
-Top bumper reinforcement COULD BE OR NOT needed. Some 73/74 still got the same 71/72 top reinforcement with the provision for one of the grill brackets, some not. They are basically the same (71/72 vs 73/74 ) but JUST that difference on SOME 73/74s what didn't got the earlier kind.
-Wiring, relay... can be made at home althought some reproductions are available. But you will save A LOT making it by yourself. Is really easy to build and just need a Bosch 5 prongs relay, safety breaker... plugs, wires and a grommet to go throught the firewall. Somebody around ( I think at dodgecharger.com board )used a Ford relay really similar on how it looks to the original Mopar one and the PN was posted on the thread, however, being the regular relay available anywhere around in the world and CHEAP, I wouldn't worry about.
-Originally diff headlight buckets were used for hideaways grill, being shorter for clearence between headlighst and doors when opening, HOWEVER have been prooved this is not mandatory. Even 71/72 non hideaways cars have been changed to hideaways and the doors still gets some clearence to open. 73/74 gets more clearence due the added bumper fillers gap, so you shouldn't be worried about that
wiring diagram ( this is one I made long time ago and has been shared everywhere ):
The inner Y brackets brace used on 71/72 but replaced by the bumper mounted on 73/74 ( both to frontal impact protection ) is the one labeled here as BRACE.
The diff on top bracket reinforcement is this, WHICH some 73/74 STILL got the provision, My 74 did, but my car was assembled in Vzla so maybe is not the best example, however A LOT of 73/74s I have checked in USA still got the earlier kind. So will depend on your own car. This provision can be also build and welded!!!
and this would be the full view assembly:
