I am not familiar with the door hinges on the 78 Nova, but you car seems to use the same bushing and pin kit that I used to replace these items in my 74 Nova. Below are the procedures I used on my 74 Nova.
It is normally the door hinge bushings that wear down and also need to be replaced along with the door hinge pins to reduce/eliminate door sag.
To complete this task, I had to find a way to support the weight of the door so I could remove the 2 hinge pins... and then move the door away from the car so I could also R&R the hinge bushings.
Before I removed the door pins, I had to remove a small spring located just above the lower door hinge.
I used a GM Door Hinge Spring Compressor to remove the door hinge springs. https://www.amazon.com/Performance-T.../dp/B0187IOXJA Note: The same type of tool is also available on eBay and many auto parts stores.
Since I was also working by myself, I tried to support the door from just the bottom, but that didn't work out to well because the door was too wobbly and unstable when only supported from the bottom.
I then used an engine hoist with a couple of ratchet straps holding the weight of the door and this worked fine. I looped the ratchet straps around the bottom of the door (one strap towards the front of the door - and one strap towards the back for stability) and raised the hoist to hold the weight of the door... I did not support the door by the window frames (I know this was probably overkill, but I used what I had on hand).
Once the hinge spring is removed and weight of the door was supported, I used a small hammer and a punch to drive out the pins... and then moved the door away so I could R&R the hinge bushings with the same punch and hammer.
Once the new bushings were installed, I moved the door back into position > installed both door pins > and reinstalled the hinge spring.
In my case, The hole the bushing fits in was also out of round so that even with a new bushing the door would still wobble and sag because the bushing would move. So I found bushings with more meat around the outside at Advance Auto that would fit in a larger hole. This meant on the driver's door I had to use a round file to make the out of round bushing hole bigger to accommodate the new beefier bushing. But then the bushing fit snug--no more wobble.
Also, two people would be very helpful since the doors are heavy & awkward.
I made a bracket that would fit my floor jack and under the door and jacked the door up slightly with it wide open and had another person keep it upright while I drove the pins out from underneath. Like they said remove the spring first. I picked up a cheap tool for that at Oreiliy Auto. If yours are not worn into the steel part of the hinge the new kit will make the door work great. My pins were a bear to drive out so soak them with something first.
A door cradle works well, this one was not real expensive and attaches to a floor jack. It allows for safe removal of doors by one person. I thought about selling it when finished with the first doors, glad I didn't it has been real handy for helping friends and for projects I didn't know I was going to have.