Anything and everything Spitfire.

Does anyone have any experience of fitting harnesses to a road going Spitfire (I'm thinking most likely 4 point, as available from Moss and countless others). My mk2 car has fixing points for 3 point static belts, but I'm wondering how I'd locate the 4th point. Perhaps a bar mounted from one wheel arch to the other using the exisitng mounting points but with 2 eyes per seat welded on the bar for the top harness straps to fix to? Alternatively, I see there are 3 point harnesses out there. Anyone with any experience of these?

Tarquin