Check out Clifford Performance. You can also pick up a copy of the Chevrolet Inline Six-Cylinder Power Manual for about $20.00.

Personally, I'd swap the six for the relatively new 4.2L inline six in the new Trailblazers (275 HP) as these can now be reasonably purchased from your local salvage yard. Just make sure you buy the trans and the wiring harness/computer with it.