the mainsheet will be a 7;1 straight foward purchase with a cleat on the traveller car, instead of tying off the other end of the sheet - run it inside the boom to a 4;1 purchase that exits the boom at a swivle cleat about mid way along.
this will give a 7;1 coarse trim for the helmsman and a 28;1 fine trim for the mainsheet man out on the float.
This is the system that have used on a seacart and it is great.Just keep the boom swivle cleat far enough foward that you dont hit your head on it exiting the main hatch!
It should also be fairly easy to add or remove purchases if required.