I have a B38 Ocean which is very similar to the 40.
I fitted an Eberspacher warm air heater back in 2000 when the boat was new. It was quite straightforward to fit but takes a good 10 hours.
The heater unit itself fits in the central space under the bunk in the rear cabin. This was the most difficult part of the installation as there is not much room but it is where the professionals fit the unit (I checked on someone else’s boat).
Fitting the ducting is easy as the holes through the bulkheads and lockers are already there. You need a good sized hole cutter for the vents into the cabins. I have 4 outlets – rear cabin, saloon, forepeak and heads using the old DL3 unit. There is now a newer (and higher rated) 4KW unit that would suit your boat.
One item that I would thoroughly recommend is fitting the control unit near the bed so you can turn the heat on in the morning without getting up.
Once installed the unit works very well. It sounds a bit like a jumbo jet taking off when it starts up but you soon get used to it.
In the UK if you specify heating to be fitted to your new boat, they are now using Volvo Ardic (I think) and not Eberspacher. I understand that works very well too and I guess installation would be similar to the Eberspacher.
A further alternative is to use a ‘wet’ system with blown air ‘radiators’. It is a bit more expensive and difficult to fit but more versatile and you get hot water as well.
Hope that helps. If you do decide on the Eberspacher and need any further advise on fitting – just ask.