i have a Gear reduction on mine. i went to GM and bought the replacement starter 1st. after an hour of trying to shove it in there i fliped it off and took it back. i then bought my current starter, which is actually made for high performance marine equipment. yep, a boat starter. but it took me 5 min to put it and i didnt have to shim it or anything. plus the gear reduction sounds cooler than the original.
i didnt get the boat starter from GM though, i got it from a local starter alternator place. price was about $160