What harness are you putting in, I replaced all but one in Mikes Sliver Mullet. Only the dash harness was a bitch until I pulled the dash out then it went in with no problem. While the dash is out clean everything in there and replace ALL of the bulbs. If you get a harness Order through Auto City Classics they sell the American Auto Wire series harness and fit is spot on for every connector and length. Only changes were from not getting the one wire alternator regulator bypass that was needed.
Not sure what I bought for harnesses, the dash gauges aren't in either, cars a empty shell now.
What you have for a harness makes a huge difference. An exact replica is usually a plug-n-play where a Painless or Autowire Classic Update kit are VERY labor intensive and take many hours to fit, terminate and wrap. I've helped install 2 Update kits and it's not a 1 day deal. Dig out what you have and let us know. It may be possible to do this as a club tech session.