Wiring harness questions

[RazorX]

So I am planning on dropping in a Gen III Hemi , not sure which year, etc.  But my 1973 Challenger needs a complete harness anyways.  It looks like the engine, computer etc all need their own specific wiring harness, so what is needed for the Hemi swap?  In other words, should I buy a complete harness and just not use the engine wiring?  Or would it more economical to piece together the harnesses, ie rear body harness , under dash wiring, etc?  What is the most economical way to go about this? And what brand is most preferred?
Thanks

[Chryco Psycho]

I have done this a few ways
We used the provided engine harness & ECU & connected up the existing harness for ignition , light Etc
I have also rewired the whole car using American Autowire which is not cheap .
If you have good wiring for most of the car adapting to that should be the most economical .