I always work on the suspension/brakes first. I was always told you can make a car go, but will it stop and steer and keep from wrecking? I do the suspension while the motor is at the machine shop, trans being built. I paint the engine compartment, interior floors, trunk and fix any rust while there are not any obstructions in the way.
I do the motor, trans, rear. wiring and get it running. I put the interior together and get it tagged. As I drive it, it does many things for me. enjoy my project, work out the bugs. As I drive it I work on the body, and upgrade the interior B/C usually I gather nicer and/or new pieces to make it nicer. I find by doing it this way, by the time it is nearly ready for paint all the changes are done and you won't be mad dinning the old paint while doing the mods needed to make it right.