Hmm Tunnel hulls,

They normally use an outboard arrangement, but in your case I'd recommend a flex cable for that, it should mean you can position the drive parrallel and as far back as you like.

You might be better off fitting a 480 if it will fit or even an Ebay brushless/esc combo if you want to spend the money as 11.v will be too much for a 400 and will cause meltdown, a gear box will be a waste of time and weight for a Micro hull.

I dont know who the best supplier will be for Oz, but a lot of the good stateside companies stock this stuff

For Ideas look at:

[link=http://www.microhydros.com/]MicroHydros[/link]