Brown sludge is pretty common in skyline cooling systems, try flushing the motor and rad 1st, its easier than headstuds
WHen i replaced my manifold coolant hoses the other day they were 1/2 solid due to the crud build-up in them as it clearly hadnt been flushed ever lol.
Washed and Vac'd my car today. Also did a few little things in the engine bay, still pretty messy though