[storm9c1]

+1 on the driver's side head for a normal flow Chevy V8 engine. On a reverse flow engine (like the 90s LT1), it went on the passenger side head.

You can also put it in the water pump or intake bung as others suggested. You might get a more consistent reading there since the heads do heat soak (and sometimes cars with EFI use two different sensors for this reason). If you are using EFI, and need a probe for the computer, I'd say use the WP or intake. If you are hooking up a gauge, I'd say put it in the head. A head will heat up very fast if there is a cooling issue and it will also show up on the gauge faster to warn you.