I have a 1999 Pontiac Bonneville SLE and want to but a series III supercharger crate engine in my car. will i need to add any wire harness and change the computer around at all. And what is the smallest pulley i can put on the engine with out any motor work.

you will need it add a few wires to your Harness

you will also need a set of the Series 2 fuel rails, injectors TB adapter and a L67 or more favorbably a N* throttlebody.

you can MAYBE run a 3.6 pulley with a decent tune

I would suggest no small then that pulley. You will need to scan for KR. If you want to go smaller you will need to look at 1.9:1 rockers or better yet, a cam.