I'd also go with a new harness. I was getting tired of chasing 40 year old wires that had been altered,etc... Was worth the money and
helped clean up the look of engine compartment quite nicely.Be patient with the harness where it plugs into the firewall,it's a tough spot to get at and most of them are on there pretty good but you should be able to get it off.
They make factory "custom" harnesses which take into consideration classic cars running applications as HEI and internal regulator alternator with the rest of the harness being factory fit so you do not have to cut and splice the harness to wire in above said applications.

David