take a 3/8" - 1/4" BSP reducer bush, bore out the centre to tapping size and retap the metric thread inside.

ECT – M12x1.5 tapping drill size 10.5mm
IAT – M14x1.5 tapping drill size 12.5mm

you will find that the M14 thread leaves a VERY thin wall section inside the 3/8" reducer, and you will need to bore and tap it central to ensure the tap doesn't break out. The resulting adaptor will be quite fragile until everything is fitted - loctite the adaptor into the manifold or x-over pipe or wherever and pinch it up carefully.