Installing headers 73 duster 318

I am not familiar with 5901 Hookers. However, if they are designed by Hooker for your car, they should fit. I have done headers in and out of my 69 Barracuda many times for one reason or another. I will give you two scenarios. Keep in mind this is from my experience, so others may have other ideas.
1. You will need at least 4 people for this one. One person on the engine hoist, one person to move the engine around as it is dropped, and one person on each side of the engine holding onto a header. It is much better if you have the car up on jack stands several inches. Make sure the steering box is installed and the starter is on the engine. As the engine is SLOWLY lowered into the car, the 'header' guys will CAREFULLY snake the headers into place. A fifth person to do whatever is needed AND to crawl under the car and lift up on the back of the tranny to help guide the engine into place. If you are installing the engine without the tranny, it would probably go smoother, but I have always done it with the tranny attached. If you have Power steering, you may well have to put a 'ding' in a tube for clearance. I did. Also, some towels protecting the inner fender skirts are good.
2. You CAN do this one by yourself, but a second person is great. I have also removed and replaced headers several times with the engine in place. Get the car up on the highest jackstands possible. You will need the clearance. The steering box will be in place and the starter will be on the engine. Unbolt both motor mounts. BTW, I'd do the passenger side first. Using a floor jack and some type of pad (I use a piece of 2x6), jack the engine up by placing the 2x6 under the oil pan. with the engine up as high as you can get it, snake the header up from the bottom. You may have to reposition the jack so as to tilt the engine towards the passenger's side (the driver's side will be higher). If you wiggle the header just right and have a helper pulling up from the top, you will get it in. Then attach it to the engine with two bolts finger tight. Same process (more or less) on the other side. The passenger's side is a lot easier.
GOOD LUCK