No idea about the harness yet. I have been reading some of the threads on the 3g swap but most are on the aero foxes do I need this kit for my 84? http://www.paperformance.com/Scripts/prodView.asp?idproduct=73Check www.paperformance.com
NVM... Here's an actual link: http://www.paperformance.com/scripts/prodView.asp?idproduct=200
They have install kits too though I think you mentioned that you have a plan for that already.
I think your plan is a good one though. Without breaking out the slide-rule, you can get everything you plan to run in place. After that, you can still add another alt and harness if it's not up to the job.