Series I perfromacne parts are more difficult to find than Series II, yes. That still doesnt mean it cant be done. Your supercharger however is completely different than the newer motors. Newer L67'* have an Eaton M90, with is a big unit and comes with a 3.8" pulley stock. Most common size mod for them is a 3.4.
Your Supercharger is an Eaton M62, which is a much smaller unit, and in turn also has a much smaller pulley stock. Not sure what the size is of yours stock, and without that its hard to judge how much lower in size you really should go.
Can you measure your pulley sometime? Side-to-side?
2004 Impulse Blue Metallic GTO
Black Powdercoated OE Wheels
Predator w/ Custom RWTD Tune
Hardwired V1 w/ Remote Display
HoŽn Highs, Lows, Fogs
B&M Rippper Shifter
GTPprix Shift Light/Buzzer Mod
Split Rear Exhaust w/ Custom Tips
1995 Bonneville SSE - Sold!