if you've got loads of time (and i mean loads) and an old hull, it's doable, it wont be just cutting the deck and sides off and putting a set of tubes on that's for sure.
the floor on the hardboat will probaly be at the wrong height and the transom will have got some of it's support from the bits you will cut off, i have seen in some hardboats that the transom doesn't go right across the back of the boat, so new transom maybe, let's go to forming the ears on the transom while we're there, you will need to know attachment flange height and tube size beforehand. will you guess the displacement of the finished boat, if you get it wrong the tubes maybe to high or low when in the water
i used to work for a rib company that used a powerboat mould to make there 10m so i know what you will encounter.
i hate to alway's agree with cookee, but if he talk's sense then who am i