IMO (having done a few of these and an about to do several more) You have 3 options
1. go through all of the old wiring, repair and replace what is needed and reinstall it
2. buy a brand new replacement harness and install it
3. buy a new after market harness and install it
So, with option one you have the following pros and cons
PROs
- you know the harness is basically there
- most of your fittings are present
- generally costs more time than money
CONs
- you still have 40+ y/o wiring
- if there are bad wires present, you may end up chasing demons for a long time
- wiring is old and stiff and could still be bad even after inspecting it
With option 2 you have the following
PROs
- you will have brand new quality wire
- dont have to worry about bad connections
CONs
- costs ALOT (like over $1000 for a whole harness)
- it might be hard to get a harness that matches your car exactly due to the many options Mopars had
With option 3 you have
PROs
- you have brand new wiring
- costs WAY less (depending on what harness you get, there are some Mopar specific new harnesses that are $600+, but there are some that are less than $100)
- every wire is labeled ever 6" as to its function
CONs
- you have to add/attach the connectors you need
- you do have to be relatively decent at wiring/electrical to make everything work as intended or you want
- the factory wiring diagram will no longer be relevant
I have done #1 a number of times and #3 a number of times and will be sticking with this one. My reason is this, first off I am convinced that Chrysler hired the bottom 5% of the electrical engineer classes from probably the 40's until for sure the 90s. They wired things up in a very confusing manner and seemingly no rhyme or reason. Of course I am exaggerating a bit but the wiring in a 70's era Mopar is definitely not straight forward; they used too small wire for some of the more important jobs and connected wires to one another (thing in line distribution blocks) that really should have not been connected (IMO).
These cars are pretty simple to be very honest and a generic new harness works very well, the only real challenge is the instrument cluster and some of the inside lighting (dash lights), you just have to gain an understand of how you want it all to work. Also, if you are going to use the stock switches, you have to spend some time to understand how they were intended to work and then wire them that way. There is some trail and error but it really is not horrible.
Having said all that, I have to admit that it does take some understanding and a few special tools link crimpers for some of the connections (like the Packard 56 connectors that the car originally came with). I have an entire electrical section in my shop and probably several thousand dollars worth of tools, testers, wiring, connectors, etc. so for me it isn't as daunting an issue.
Probably not the answer you were looking for but... BTW the vendor you are looking for is M&H wiring
http://www.wiringharness.com/ 
