The LS engine uses front timing cover and rear main seal cover made from cast aluminum. Each has a seal pressed in to keep the oil on the crankshaft from leaking out. Between each cover and the engine block are seals made from stamped aluminum impregnated with a rubber seal. Aligning these covers is critical to ensure there are no leaks. There is a set of simple tools that make the alignment process very easy. Sac City Corvette sells a complete set of Alignit tools that will work on every LS engine. The three-piece kit includes a rear main, front timing cover and simulator tool.
These pictures show the simulator tool. This aligns the front cover with the seal installed.
The Alignit tool kit is available here: http://www.saccitycorvette.com/AlignItTools.html