Your heaters installation manual should tell you all of this. It certainly does for my Eberspächer, with maximum and minimum lengths of all the ducts and pipes and where to place the fuel pump. It may be you only have the operators manual, so have a look for the installation manual which should have those details. Or tell us which heater you have, as the installation is manufacturer and model dependent. You'll also want the one for marine installation, not vehicle one, as there are some extra things to pay attention to, such as looped up exhaust/intakes to prevent waves coming up into the heater.
Fuel take-off should be not all the way from the bottom of the tank like an engine take-off would, so that you never end up being unable to run the engine because the heater ate all your fuel - about 80% down is more like it.
As for running ducts through the boat, dig up all your lockers and have a look where you can run the ducts of appropriate size without taking up too much locker space, keeping in mind that these ducts will get quite warm (so don't run it past the fridge, for example). Also keep in mind you'll have to fit a drill with a hole cutter in there. The ducts are relatively flexible, but will not take sharp turns well and are liable to damage from being crushed.
