'74 Duster ?'s

#1 What is it out of? The casting number and part number should be 7 digits long.
#2 No, but what does it hurt to do so?
#3 You probably won't know that until you get the engine and headers in the car. All header companies aren't the same.
#4 What 70DartMike says except you won't need to change the upper control arms. You already have the big ball joint type. You'll need new wheels though....and a disc brake master cylinder.