A lot depends on the hull
geometry as it is now, and I could not give any specific advice unless I had a set of lines available. And that's the first thing to consider--the new hull
lines. Done properly, the new extension will be adding more weight to the boat, and you want to counter that with more buoyancy to support that weight. Add too much buoyancy, and the new configuration will cause the boat to trim more down by the bow. Add not enough buoyancy, and the hull will trim more down by the stern. If the boat trims adversely now, such as too much trim by the stern, then you can design the new extension to correct that. The weight of the structure and the buoyancy of its shape all have to be considered, designed, and engineered together so that the trim comes out right in the end.
The new extension is also going to affect the stability of the boat because of the new submerged volume and weight change. All of that has to be tracked as the changes are made. It is most prudent to measure the drafts of the boat at the bow, stern, and amidships before you take the boat out of the water
so that you can better judge and decide about changes as you go along. Presumably, you do have hull lines. If you don't, that makes doing a smart and proper job all that much more difficult.
As for steering
, if you are not moving the rudder
, then you do run the risk that steering capability is going to diminish because the submerged hull behind the rudder is causing extra drag, and so steering authority is going to diminish. The correction for that might be a larger rudder, but that impacts the size of the rudder shaft, the rudder tube and bearings, and the size of the steering gear
. All that has to be taken into account.
6' to 8' on a 40' hull is a 15% to 20% increase in length--that's a lot, and so the advice above about seeking professional help is well given. Do so, because it will be easy to make mistakes
if not thought through properly.