Jim, Something like 20 years ago I replaced the engine harness on my '67. I was fairly clueless and it still went pretty fast - maybe 1/2 a day - like 4 or 5 hours at most.
Almost every year and body is a little different, but everything for the engine harness is pretty accessible. It will prob include the headlight circuit - on 67 that means removing the front grills. I assume 66 will be the same. - you know these cars well - it should go well.

ps. save the original harness. I've raided mine for reference and parts in time since the replacement. Not every detail on the YO/MH harness is 100% correct so its good to have the original - it will roll up into a box pretty small.