Brake conversion confusion

I am doing the dual brake conversion on a 66 Barracuda. I have a new dual master cylinder for a 67 Barracuda but there isn't a shaft that attaches to the pedal. The single master cylinder I have has the shaft but it attached to the plunger. So am I supposed order a shaft for the dual master or cut the shaft on the single master cylinder or something like that? I couldnt find any information about what shaft to use doing this.
Thanks