Been through all of those. The real root of my issue was having the wrong motor mounts ever though I have the correct BBC frame stands. That's been fixed and the engine sits a bit higher because of it. Some of these work with the wrong motor mounts installed.
I ran into trouble with the drag link hitting when I installed a finned aluminum pan. Even though Impala was listed in the application, it didn't quite fit. Milodon makes a pan that will fit well (#30970). Cleared with wrong motor mounts, tons of room with correct motor mounts. Still think they hang too low.
I'm on my 3rd set of headers to get a good fit. Hooker long tube headers fit without the need to dent the down tube around the steering box but the ground clearance was too low on my lowered car. Cleared with wrong motor mounts, would have ~ 3/4" more ground clearance with correct motor mounts.
Hedman 68618 mid length headers did not fit. I had to beat the crap out of the downtube around the steering box. They "worked" but certainly not designed for the car. Made to work with wrong motor mounts, I don't think correct mounts would have made much difference.
Sanderson BB10 headers fit. I can get a finger between the header and the steering box with no denting the down tubes. Clears my clutch linkage. Easy spark plug install, easy access to the oil filter. Cleared with correct motor mounts, maybe clear with short mounts or minimal adjustment. The attached picture is of these.