Hello John,
Many thanks for the comparison diagrams which are very helpful. The longer hull of the Finnjet is not a problem because I would probably keep it that length and just alter the funnels (one instead of two and in the approximate midships position) plus build up the sides and fill in the stern deck area, thus giving the model a fuller look. My intention is not to replicate "Stena Europe", etc, just to draw strong influence from it with a similar livery and other features. However, if anyone does know the answer to your question about how to reduce the length of the Finnjet hull whilst maintaining enough room for the RC gear plus keeping the model buoyant I'd be most interested to hear! John what components for the RC are you using and where did you buy them all?
Simon