a rib to big?
with that size engine i think you would be better off with a smaller rib, I think with a 40 on the back of a 6.5m rib, you will struggle to get out of the water, you would probably need a finer than normal pitch prop to get out of the hole, giving a low top speed and poor economy. for a 5m rib two 40s would be about normal.
i am sure someone with more idea than me can point you in the right direction, but i would have thought 4.5M would be better suited to your engine, I had a 5M hard boat which is lighter then a rib by far, with a 40 and it only made 25kts. it took for ever to plane too.