For any old-school LS 400 owners handy with a torque wrench and a willingness to get their hands dirty, there is no site better than Lexls.com:
This site is jampacked with a range of useful information, with everything from changing your oil to replacing your timing belt/water pump. Each tutorial is extremely well written and features photos of every step along the way.
This site gets my highest recommendation, it’s a labor of love for the author, and it shows on every page.
(Owning a first-gen LS, I’ve used the tutorials for a few small things, like cleaning the air filter, but because of time and space constraints, I leave most of the other stuff to my trusty mechanic.)