I tried to cut and splice the old harnes while swaping an LT1 into my 67 camaro. After three weeks with schematics, spliced wires, and tired eyes; i came up with just enough reach in the harnes to make it to the fire wall, but not through to the other side.
I ended up buying a harnness with 3ft extra reach which put the computer under the back seat. Worked great.
Save your time and buy a predone harness.
m.02c
My harness came from Street and Preformance. Real nice harness.
PS i know my spelling sucks, bear with me.