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The bare minimum between a HD7698P/HBU55 and a FM6 would be 5’ (That roughly puts the HD7698P/HBU55 at the edge of the FM6’s capture area. Ideally it would be 10’ (That roughly keeps the capture area for the HD7698P/HBU55 separate from the capture area for the FM6).
A HD7698P or HBU55 will be better on FM than no antenna at all, so you may not need a FM6. At the moment the FM6 is the only commercial off the shelf mass produced directional FM antenna I know of (with the exception of a channel master model that I think is overpriced).
In general chimneys are not designed to hold antennas. The wind load from antennas combined with the lever-arm effect from a long mast can be enough to damage chimneys. I have no experience with chimney mounts. I believe the rule of thumb is anything 10’ above the last point of support needs to be guyed. Chain link fence top rail makes a good mast. It is inexpensive and quite strong. When in doubt, call in a professional. A good antenna installer will do engineering calculations for wind load to get the mount and mast right, and offer a no-leak guarantee for anywhere they need to penetrate the roof.
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