It depends on how far you want to go with the car. Doing a full tare down and rebuild takes a lot of time and money! You have to stay focused for sometimes years!
If you are up to it I would tare it all down, strip the old paint so you know what rust/damage there is to fix and do so before making any mods, ie, mini-tubs, sub-frame connectors, ect. Then mock up as much of the car as possible including drive train. Tare it down again. Body and paint is next. Final assembly and working out the bugs of a totally rebuilt custom car.
Or, you can take a driver and make changes as you enjoy the car.
Either way works, you just have to understand what works best for you and go from there!