70 dart 340 wiring harness

Get a Service Manual.
The pdf scans here will at least get you started in most cases.
MyMopar - Mopar Forums & Information - Service Manuals

Go to the back of the electrical section and you will find the wiring diagrams.

Many TSBs can be found in the The 1970 Hamtramck Registry Library Page (1970 - 1974)

And lots of Chrysler Tech tips here
Master Technician Service Conference - Chrysler's Training for Mechanics

Including a couple about reading wiring diagrams.