I never seen that hinge kit before I am assuming looking at it that its for the trunk?--Looks bulky but its hard to tell in photos with nothing for scale.

for a trunk release I have an electric release from a late 70's early 80's Mopar that I will use in my 39. Simple two wire setup and you can pick them up pretty cheap ($10 or less usually).

Firewall, with something as custom as what you are building I would recommend one of two approaches 1 - fill all the holes and drill out what you need when you get to needing them so they can be placed where you need them. 2- cut it out and start fresh, this allows you to recess the wall if needed for the 5.7 and provides a cleaner look because all the factory bends and ridges are removed.

Window and door lock kits are really a personal preference there are tons of economic options out there. When it comes time I planned look at some of the other websites dedicated to this era Plymouth and see if anyone has discovered a common modern retrofit for the window and wiper regulators (I assume you will run wipers).

With my 4-dr I will be leaving the external handles on the doors so I am not coming completely smooth. If you plan on shaving the handles I would say get the bearclaw latches and some good soliniods and wire it for poppers with a back up manual release.