The only experience I have in a full replacement is with an old lifted 78 Chevy 1/2 ton truck. I layed the entire thing out as it would basically go into the truck but next to the truck and just started pulling wires through the firewall. Just ran it neatly and plugged in as I went. Painless is the wiring kit I used and was as simple as simple could get as far as rewiring the whole thing.
Not a bad investment at the time considering the tail lights would only work with the dash lights out, hit a bump and the headlights would go out and the blinkers would work, or hit a bump and the headlights would go out. Then the interior lights would turn on whenever they wanted. That truck was only a toy and only saw road on the way to go mudding but it was the back up for my 71 chevy LWB that I used to race. Worst of both worlds there, I don't think either one was drivable at the same time.
But then I got married, had a couple of kids and couldn't afford to keep both running for what they were built for. Life happens. Which is why I bought my 66 project to build as a stout, powerful, potent and reliable 5.0 powered daily driver.