Hi Steve
Attempting to silver solder the flexishaft will result in it's destruction. The heat required anneals the strands of the flexi rendering it useless.
To soft solder, heat the brass ferrule with the fluxed flexi in place AND STRAIGHT (you may need to make up a holding jig with the ferrule at the bottom and the flexi upright). Use a pencil flame on the ferrule, and when the heat has spread through to the flexi, feed the solder in at the top. Capilliary action will draw the solder down into the ferrule. You could LIGHTLY 'tin' the flexi with solder before you put it into the ferrule, but you may have to carefully file off high spots (making sure it's straight) before it will go in again.
A number of European boaters apparently 'Loctite' the shaft in (not sure of the type number). If I were 'glueing' it in, I would prefer something like JB Weld, as the strands present a limited surface area to the ferrule, therefore less contact for some types of adhesive.
Good luck with the Shockwave.
Danny