Where to Start
I think everyone is in agreement here. Work on upgrading your brakes and suspension before starting to add more power. Better brakes and suspension will make the car more enjoyable for cruising while you work on building your drive train.
One of the first things I did was replace my leaf springs, torsion bars and shocks. I completely went through my stock 9 inch brakes. I also welded in some sub-frame connectors. The difference in driving was like night and day. Now I am upgrading to disc brakes and replacing all of front suspension parts. Next winter, I will work on the powerplant.
I would be very careful letting ANY untrained driver behind the wheel of a 40 year old car. They do not behave or react like a new car. Just my opinion though, do what you feel comfortable with.
Enjoy your Dart and welcome to FABO!!