This occurs in the Custom Cubic overworld. The spout has a bottom (it terminates at that island), but does not have any top that I can find (hence the "+" in "kilometer+") It does not appear to be infinite, because when I teleported 10k blocks up it was not present, but because it takes several minutes for blocks to start loading after a teleport I have been unable to find the exact top.
Screenshot:

preset:
Exa-Islands-821-no_ores.txt
seed: 7171866076957742794
coordinates in screenshot
Fml-client-latest log:
fml-client-latest.txt

Note: The spout is weird and lumpy because it has not finished flowing yet. As I move further up, it updates the spout and it starts spreading out, I believe that if it was allowed to finish flowing it would be uniformly as thick as it is at its thickest point in that picture

You enabled generating modded stuff in cubic chunks world. You do it at your own risk. There is a reason it's not enabled by default. Mods are free do do any stuff they want here. That includes generating HUGE stuff.

No they couldn't. This generator is not inteneded to be compatible. It's not supp;osed to be used they way cubic chunks uses it. What cubic chunks does here is a massive hack used in hope some stuff will work out just fine.

@AlexGreason try to create issue in BuildCraft issue tracker. They could fix that.