Thanks for the input guys. I ended up using the zabel filter as it fit right into my concrete baffle in the tank. I also left the stone in the gravel filter box – the new guy that I had pump the tank out also cleans the gravel filter box out at no additional charge!!!! I will treat the zabel as a primary filter to be cleaned periodically and the gravel filter box will be a secondary protection against the flow events that occur when cleaning the filter. I also plan to block off the tank outlet when removing the zabel filter to avoid any solids from exiting, but even if they do, the gravel box will catch them.
Thanks again for the advice.