How much do you need to panel the back end underneath...I'm thinking in particular of dust/muck/spray getting into things in the boot.
I've roughly panelled some of the bigger holes but wondering how far one needs to go towards a fully sealed bootspace.
Thx.

A potential problem with sealing up the rear end is that you will end up trapping all the heat in from the diff and that travelling down the transmission tunnel etc. You could end up overheating the diff if you are too effective with this

The covered underside on my LA has some very nicely arranged holes to vent in some air, so the heat build up isn't an issue. Might be worth checking with Matt @ Procomp as he originally built the car.

Good points!
I was thinking of all the fiddly little gaps, like between the 45degree members outboard of the tank and the rear panel sides; under the tank (or the 2" gap between the tank and rear panel and the nadgery gaps behind the tops of the rear springs.