If you beam the boat up with a new beam front beam and the old rear beam, the front will bolt up square and the rear will bolt up splayed outwards so there will be twist in the load on the hulls. If anodising will break your budget don't worry about it but make sure you use duralac or similar on all rivets and bolts to impede corrosion. A cheap place to look for this type of pipe is a Centre Pivot manufacturer or similar for agriculture, you would just buy a length and cut it up