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I would say that if it's not a factory Ford coil, I would not mount it laying down anywhere if it could be avoided. The originals were better than most aftermarket ones (especially cheap ones) at filling with the cooling oil. Some will overheat more easily when laid down flat like that.
When I get new coils now and need to keep the stock "canister" style, I buy epoxy-filled rather than oil filled. MSD and Pertronix are two popular ones. They don't care what position they're int, take heat and vibration in stride, and look relatively stock.
If you need to keep that style and have an oil filled aftermarket version, then maybe over on a fender or somewhere that it can be mounted in a more vertical position, and just use longer wires? But yeah, if you need to maintain all the original wiring lengths (and most of us likely find that much easier) then what they all said about mounting points. Just be aware of some of the limitations mentioned.
When I updated my ignition on the '79 I ended up with an e-Core style coil (think later EFI models) so just about any surface and any position was no problem to mount it on.