I think I would do an aftermarket harness if you really are unsure. It will probably make the difference between really hating this project and really enjoying it. The new harness will have everything labeled as to where it goes or needs to be terminated.
I have had good luck with Robert at
www.fuelinjectionconnection.com for harness work. A new stand alone harness and reflash of your PCM is probably going to be close to $500. To get the speedo to work you will either need to swap gauges and get an electric speedometer, or pick up a Cable-X box. Those are about $300 or so.
While you are removing stuff, I would suggest removing the EGR as well. Almost all of the LT1 cars I have had have had EGR issues. If you are thinking of running a HEI, that may be in the way as well. I personally haven't had any optispark problems, although some have. If you go with the later, vented version it cuts down on a lot of the moisture problems. Having teh engine out makes it a lot easier to swap in a new water pump and Opti as well.
The LT1 makes a pretty nice swap. I had one in a 79 SWB, and I really liked the way it ran. Good power, pretty good mileage, and the "gee whiz" factor when you open the hood were all good things in my book.