From an older interview with Donner, Puzo wrote a long treatment, really long, but still just a treament and not a draft. Like I said, Puzo was just onboard as a name to raise the money due to his Academy Award/Godfather status. He basically found the details of Superman, backstory,etc, and wrote a treatment to say he actually did something, collected his money, and left. It was also much more graphic than what eventually came out, with Lex Luthor killing people, etc. The Benton/Newman tratment was the goofy one. That was the one that the original director, who had directed a few of the Roger Moore goofy Bond movies, oversaw. When Donner got hired, it was the Benton/Newman screenplay that he was given, which had all the camp and was too long. That was the one that Donner said if he was going to direct it, he wanted to redo the whole thing. So he and Mankelweitcz looked at the original Puzo treatment, liked a lot of stuff in it, and it's sesrious take as opposed to Benton/Newman's goofy campy script, and wrote their script to be alsmot as serious as the Puzo treatment was. But, Donner said they also had to tame down what Lex did in Puzo's treament as he was basically offing people the way people were getting killed in THE GODFATHER. It was as serious as the Godfather. Donner knew this would be a movie marketed to kids as well as adults, and so that is where campy Lex and otis came about.
Now, I believe the draft that is credited as the Benton/Newman draft is actually the first Mankelweitcz draft, but due to the rules of the writers guild, had to be credited to Benton/Newman. There is much mention that Benton/Newman got total credit for everything that Mankelweitcz did due to writers guild rules at the time. Hence the Creative Consultant title in the opening titles. My reasons for believeing this:
1. Mankelweitcz said he came up with the line "This is no fantasy", the first line Jor El says, on the DVD. That line is in that script credited to Benton/Newman.
2. Donner and Mankelweitcz mention too much camp in specifics from the draft handed to them, which was the Benton/Newman draft that the previous director had worked on and planned to shoot. Both Donner and Mankelweitcz mention specifially a scene where Superman is looking for Lex, sees a bald guy, swoops down to get him, and the bald guy turns around and it is Telley Savalas saying "who loves you baby". That is not in the draft credited to Benton/Newman, and it smacks of the stupid one liners that director previous to Donner had put in the Bond movies that he directed. That is only one campy thing of many campy things in that script, but both Donner and Mankelweitcz have made mention of it repeatedly since Donner first broke his silence on it in the 1980 Starlog interview about it all.
I believe that Mankelweitcz wrote the first draft, to get a beat on teh story, which due to the writers guild rules, is credited to Benton/Newman. The first draft is always for the writers to find the story and to write whatever they want. But then the producers and the accountants get to it and decide per a figure of formulas how much the film is going to cost, and things are tamed down, changed, or jetisoned to get something that will cost in the budgets range. Then the shooting draft, which is what they decide to shoot after a budget is accepted to what is in the draft. An example of this paring down things from the first draft is Clark building teh Fortress himself. In the first draft, he cuts walls of ice out of the artic with hsi laserbeam eyes, then picks them up and puts them together to make the fortress. They could not do that cheaply in those days, if at all. They maybe could have done it with fored perspective and hanging miniatures, but what I read, and the time they had to do it, I doubt it. It is cheaper for a miniature set of ice spires to rise out of the water building itself.
I don't believe anything of the Puzo draft, nor the Benton/Newman draft has ever seen the light of day. Puzo's treament is not made the light of day due to contractual reasons, and the Benton/Newman one as if it came out, people wouild know Mankelweitcz wrote it all. The same thing happend with Joss Wheddon and the original script writer for Speed. Joss came in and totally wrote that guys work, and it is pretty much completely different from what his draft was. But, due to writers guild rules, that writer took full credit for everything that Wheddon wrote. Wheddon even is quoted in an interview online talking about it. He even cofrtonted the writer and asked him why he took credit for his stuff, and the guy told him essentially because the Guild rules let him. So when you see the Speed script online, it is credited to the first writer, but that is solely Wheddons script.