Yes, LT77 top housing will work. You just need to plug the reverse switch plunger hole.

You can use the complete disco drivetrain, but get a R380 defender conversion kit from Ashcrofts, and use the top housing off your current LT77 box. http://www.ashcroft-transmissions.co.uk/diy-rebuild-kits/r380-rebuild-kits/selector-rail-adapter-yolk.html