You will simply need to adapt your old sending units to the new engine. I believe the engine temp and oil pressure are the only ones that will need to be adapted.

Assuming you have a V-8, your engine temp sender is on the driver's side cylinder head between the first and second spark plug. The oil pressure sender is behind the distributor. Adapters are available for each since the LS engine is metric and the Gen I small block is standard.

There was a thread on here about turning down and rethreading an existing temp sender to fit the LS's smaller hole for the temp sender on the rear of the passenger side cylinder head.