Rear Brake Upgrade
 

ZZP offers a 12" rear brake upgrade (https://zzperformance.com/collections/3800/products/12-rear-brake-upgrade-kit), and a 2000 Monte comes with 12" front brakes. If I upgrade the front brakes, would the old 12" fronts fit on the rear? The ZZP upgrade doesn't say it includes adapter plates for the 12" calipers to fit.

I THINK the offset on the rear rotors is deeper, can't remember though. I'm on the other side of the planet from my Monte right now, or I'd go check. Not sure if you already got an answer to this either, since you asked in February.

The front will not work on the rear, the rear rotor is designed for the parking brake shoe to go inside the rotor. If you use the front it would probably not fit over the parking brake. While I never held the front and rear next to each other to confirm, that's my guess that they are definitely different, and even if the outside dimensions are the same the inside machining is a lot different.

I know the rotors are different, I can just buy 12 inch rear rotors. I was asking if the 12 inch calipers from the front would bolt up to the caliper bracket, assuming the offset of the rotors allows for that.

I dont know for sure off the top of my head, but it should be easy to check real quick with some calipers once you've popped the wheels off. Should only need 3 or 4 measurements to confirm if it's even in the realm of possibility.

I think its pretty unlikely as I've never seen it done, granted the GP group (who usually seem to be the Guinea pigs due to the sheer volume numbers) is a bit different as their rears went back to the integral caliper style (not in the rotor center) from 04+ IIRC.

Assuming it bolted up there are a few things I'd be concerned about:
-Distance from the bracket mount to the rotor (offset as you'd called it).
-Sufficient piston stroke - not sure what 12" rotor you plan to use, but the rear rotors are typically thinner than the fronts. A lot of them aren't even vented, they're solid core so they're way thinner.
-Pad contact area. Assuming you planned to run a rear rotor, the pad height is generally much smaller than a front rotor. You wouldn't really want to run with a bunch of pad hanging off the edge of the rotor contact surface.

TBH, I wouldn't worry too much about rear upgrades, they dont do a ton on Wbodys anyways. Stick a performance pad in there with some performance tires and that should be good enough unless you're planning to road race it or something. In which case you should really go aftermarket all around anyways.

I believe years ago, I read about someone who upgraded front calipers to F-body ones and to balance the looks, upgraded the rears to something a little bigger. I honestly do not remember where I read it. I want to say they used the front calipers from a 99-05 Grand Am. But again, not super sure.
I agree with Bumpin, rear brake upgrades are not terribly critical for a daily driver. But I do understand if this is to balance the look (as the factory caliper looks tiny).

For a bigger rear brake to balance the look out, I'd just stick GP GXP rears on there. Larger rotor in both diameter and thickness and a clearly documented path on the install.