Think that's roughly what I had at the start, albeit with 16" wheels. Could barely turn 5500rpm shaft speed before the vibrations near took the car apart( 1 piece ), although my overall length was about 57"

When I first fitted the axle to the car I used a modified Toyota Supra shaft as was using a Supra gearbox at the time. It was 2 piece and much larger diameter/joints than my original Ford item.

Although not sure how it would compare to a Cossie shaft.

If you put your tyre size etc into a gear ratio calculator. Critical speed of a 3" shaft 46" long as per table above would work out at roughly 160mph road speed.

A smaller diameter 1 piece would be lower. So no way will a 1 piece be an option IMO.