I agree, it is doable and the wiring harness rework info is out there, just do your research.
Best way to go on the modern swap is buy the entire donor car....pull the drivetrain, nearly the entire wiring harness, ECU, instrument cluster, and fuel pump out of the tank ......and then part out and scrap out the remains of the donor to get some of the donor cost back for the swap project.
You're not going to transplant the instrument cluster into the mustang and ruin that vintage dash vibe, but keeping the cluster allows you to test the wiring connections during the swap.
The Mustang set-up is the only way you'll get a modern Overdrive manual trans, if you want to have fun and go that with instead of an automatic transmission.
A tank set-up kit like that is a good way to go if you don't want to try to piece it all together yourself.
Even if you don't do the modern swap and just do a carburetor engine, i would strongly recommend you buy a complete new wiring harness for the car. It will save you a bunch of frustration down the road.