You don't need to compile fluxbox - there is a fluxbox package for VL5.8 alredy in the repository. Install it, create a $HOME/.fluxbox directory and then reboot X and there should be a dropdown (left side) of the KDM login manager screen where fluxbox will appear as an option.
I don't recommend blackbox as it has not been developed for about 3 years now. fluxbox as you many know, is a fork of blackbox and does the same and more for little cost in extra resources.
I compiled openbox quite easily on my 5.8 machine, works fine. I did not make a package however, so I can't send you one or I would.