This is the end of the "Mad Group" story, yes. The plan was to do more stories set in the same post-apocalyptic world (hence the whole "Part 1" thing) but Shonen Gaho, the magazine where all these appeared, ceased publication in 1971 when it was merged with Shonen King due to declining sales.
Hino did a couple more "Tomorrow's Hell" stories much later in his career and I will translate at least one of them soon. It is included in a different collection, though.
As for this book, there are 3 more stories in it but they are completely unrelated to the world of "Tomorrow's Hell" and were likely only included to get the volume to a publishable length.