Stewart
My commiserations etc
Worst thing you can do to a car when in deep water is start it.
It maybe worth leaving the battery disconnected for a while or remove the Main fuse in the fuse box so that none of the electrics are live (some are) so if there was a repeat and the ECU compartment got awash then no damage should be caused other than needing to dry out.
This is basically correct. Many car insurance companies on hearing the words, 'flood damage' quite likely will write the car off and not budge from doing so. At the moment, since the car is immobile and hard to get at (I know this is not strictly true) you have a good reason not to say anything at present.
I would suggest you find a nearby friend who will give you garage space in a dry brick or wood garage and allow you to run a dehumidifier. Then as soon as the flood waters recede, recover the Morgan to there - do not attempt to drive it.
On a trad, the bits that will suffer from water ingress first are the chassis cross-members as these are not sealed. Certainly with 70s chassis, there are no drain holes, but you'll not weaken them appreciably by drilling one per cross-member. When the water has all dripped out and the car has been dehumidified for a week, squirt in some waxoyl, fit some plugs to the new holes and all should be well in that respect.
Remove all the carpets. In theory, a car carpet should be waterproof, so washed and dried out might be ok, assuming no shrinkage.
Battery. Don't know where yours is. 4/4s 2 seaters and Plus 8s of the 70s have a low one and I could see water getting in here. 4 seaters though have it on the top of the bulkhead so will be unafected.
Brakes - dried out all the wheel parts hoses and pipes ought to be fine but I bet there is some slight corrosion on the disks and drums. As long as you get them dry soon, the first time the car is driven will get the corrosion cleaned off.
Oils - I'd change the engine, gearbox and back axle oil.
Petrol. The tank top ought to be high enough for no water to have got in, but I'd be trying to pump some fuel to make sure. Two gallons of Super Unleaded is my standard to dump in the tank for cars that have been standing or stored a while.
Engine - should be ok from what you say, but I would suggest you turn it over by hand with the spark plugs removed and poke kitchen paper down the plug holes at TDC to make sure no water has got in. And only then go for a start.
Not really as much work as you might think, and given insurance company attitudes, I'd be very wary of making a claim, or even letting them know.
Disclaimer, I think I've thought of most things, but no warranty or guarantee is given with the above post and it assumes the reader is competent with car mechanics. 
