I own a 1997 Bonneville SE with an L67. I'd like to put a 3.4 inch pulley on it. From what I understand, switching to a smaller pulley typically requires supporting mods. I thought I saw a kit awhile back that came with pulley, belt, colder plugs, cold air intake, and filter, but I can't find it anymore.

Are these additional modifications necessary? Does anybody package a kit containing everything needed? Do I have to modify the exhaust too?

that kit sounds like a kit intense offers. yes you will really want to to the other mods before or at the same time as dropping the pulley size. also if you do headers that will also improve the power of the mod

you will also want to get a tune to your PCM before and after dropping your pulley to make sure you have no KR (knock retard) which can kill your motor

A 3.4" SHOULD be fine on that year of Bonneville, but make sure you get a scan tool and scan for KR so you aren't damaging the engine. A 3.5" would probably be safer unless you plan on having someone tune the car as well. Definitely get the colder plugs and CAI on before dropping pulley sizes.

My advice, don't go smaller then a 3.5" More engines have been wrecked with a 3.4" pulley then any other mod.

To run a 3.4" here is what you will need. A scan gauge. Any time your smaller then 3.5 you should be scanning for knock retard. A shift kit, yes, sounds funny, but it does help a bit. A performance tune, even a canned tune will help. Where are you located, might have a tuner near by. High ration rockers, like 1.9:1, or better yet, a cam. And some kind of exhaust mod. PEMs are cheep and easy, but not real good, better then nothing, PLOGs, better then PEMs, or headers, best option.