So I laid out an old '74 CIS and a '73 MFI harness side by side for you.
There are a number of differences in wire length, color coding run lengths and locations of branch circuits to devices.
Other than re-using the 14 pin connector, reverse light connectors and maybe the main red power wires you might be better off starting from scratch.
Knowing what devices you are going to use in the MFI would help me to give you directions.
Here are some pictures for you to look at.
The CIS harness is the old one, the MFI one is a new clone.
Sent from my iPhone using Tapatalk