This is nothing but pure crap in my opinion. I think the feasibility studies were done in the mid-60's.

Some were, no doubt, especially the serious ones intended to explore different long-term plans. But that doesn't necessarily preclude that Rocketdyne and other companies may have done their own studies earlier. The date Kaysing gives for his report is 1959. But unfortunately we are not permitted to examine the report itself. I wonder if Rocketdyne would release it, if it exists. It's not likely to be classified, and they wouldn't give away any trade secrets by doing it.

The problem with any feasibility study done in 1959 is the amount of guesswork that would have to go into it. At that time manned spaceflight was still unrealized. Large-scale boosters had yet to be designed and built. Not much was known about the moon. The authors of such a study would certainly know how confident it was, and how much of the final opinion was based on guesses or estimates that could change radically. But a layman like Kaysing would not necessarily understand what the conclusion depended on.

Whether the report existed or not, it's not unreasonable to suppose that a dismal opinion was floating around Rocketdyne in 1959.