The Belltech spindles are the traditional type of spindle with the shaft that sticks out plus separate wheel bearings that you pack with grease and assemble. The 'Blazer upgrade' everyone talks about uses a 'knucke' and a unitized 'hub/bearing assembly' that bolts onto the knuckle. The Belltech spindle is the former style and guess what? Your 1987 Monte Carlo rotor/hub assemblies, bearings and calipers will assemble onto the Belltechs. This is a not a 'brake upgrade', this is a 'suspension lowering' upgrade. The 'Blazer upgrade' updates the G-body technology to the newer packaged hub/bearing assembly that allows use of bigger Camaro and Corvette rotors and calipers with only an adapter plate. Bigger diameter = more clamping surface = stronger braking = upgrade. If you want to keep the Belltechs, you need to obtain some hubs without the rotors (aftermarket or cut-off OEMs), and adapter plates (I'm sure they are out there, but I don't know sources. The 'Blazer brake upgrade' does not in itself lower the car.
This is all correct.... for the Belltech 2100, he picked up the 2102 specific for the 3 bolt hub
ETA: You can still put big brakes on a standard spindle, dont have to use the blazer. The stock blazer stuff is itself an upgrade though from the stock g spindle, slightly more caliper volume, slightly larger rotor, and the clamping force centerline is moved out further. Plus when doing most guys go to at least new rubber lines, if not stainless ones.
ALSO ONLY USE 2WD HUB ASSEMBLIES.
I know most of us know all this but this thread will probably come up in future google search results etc.