Ignition Coil Relocation - 1972 Duster

Unless you put 4 inch screws in the coil mount you won't come anywhere near the heater core. (the box yes, but not the core)

The one thing about can coils is that they are filled with coolant (a light oil) and when laying down can leak that coolant out and burn up inside.
This is one of the big pluses for E and C core coils, as they are a dry style.
Anyway, the point is it's safer for the coil and it's reliability to be upright.