Are the splines that slot the gearbox and prop together Alloy or Steel?

Changed the clutch on the rotten racer, then had to make a bracket up for the rear of the prop as the original was knackered, put the prop back on and the bracket snapped, remade the bracket, prop back on again, had it running and the prop moved with the bracket tight.

Want to know if we can weld the prop before a weeks worth of extra work is sent to the scrapyard in the sky.

Steel as it rusts brown I'd assume.

Definitely steel. Presume you are talking about a 340 (with the reference to alloy)

340s have a hollow alloy tube for a drive shaft but the couplings are steel. 360 has a narrow solid steel drive shaft supported by bearings inside a torque tube. Both use splined clamps to connect to the gearbox.

The driveshaft coupling clamp is a fairly precise bit of kit, obviously I have no idea of your expertise in 'remaking' this but if the original parts are not torqued accurately they will not be tight enough (overtightening strips the threads reducing the force) knackering the splines, the 340 also needs to be in the right position to ensure even overlap front and back. Usually its the 340 driveshaft side that gets knackered, needing a replacement driveshaft, and such things are like hens teeth. Sometimes the alloy tube will become detached from the flexible joint around the coupling, especially if hammered to remove.

I'd try and get another bracket. If you weld it together you'll probably damage the rubber that bonds the female insert on the prop to the aluminum tube. If the prop survives you'll have no trouble selling it for more than the scrap man would give you on here!