Had anyone updated to new wiring harnesses?

Unlikely you can find a new harness for 1963 since those had fairly unique bulkhead connectors - 1 full width, 1 half-width. On the upside, you have fewer wires and the thick ALT and BAT wires are via thick buss-bars which don't melt like most years. If you want more under-hood wires (AC, washer pump, ...), consider installing a 1965 bulkhead, since it has 2 full-width connectors, plus the buss-bars. I put that in my 1964 (doesn't have buss-bars). Buy an under-dash harness (ebay) to get the connector. B and C bodies work. You will have to rewire some of that since your push-button dash is different. I would put all new wires on the engine side, since those are usually degraded. But, a great time to upgrade to relays and such. Indeed, I installed a relay/fuse box from a ~95 Jeep in all my old Mopars.