350,000 miles!!! I heard that the LS engine is built to last and it seem so in your case. The main issue with the engine is just replacing leaky gaskets, however the the engine internals are usually trouble free if there is no excessive wear from running the engine in low/burnt oil conditions.
The LS engines are designed to run a long time if you keep it properly lubricated and fix leaks quickly. The same concept apply to the differentials and transfercase as well.