I have a 1989 Firebird 2.8L mpfi and wanna know what would be need to be changed to swap it to a carburated version, I already have the intake.

 

You need a way to dial fuel pressure WAY down. You can get a 3 way fuel pressure regulator for that, look in the carb section, it's been covered before. I used one for my engine when I went to a carbed V8, it's a Mallory if memory serves me correctly. I assume you still want everything inside the vehicle to operate as normal, cluster and all so you'll have to modify the wiring harness. You'll also need a way for the fan to come on if you're keeping the electrical fan. It's been a little while since I did this so I'll have to think back to how I did mine.
Can I ask why you'd rather have a carbed 2.8 than fuel injected? It would seem to be a step backwards in regards to the lil bit of HP these engines already have.

 

So if I remember right, I had my fan hooked up to a toggle switch. Bad thing about that is you have to remember to turn it on, I don't recommend it.
You can leave your sensor wires hooked up since they run to your cluster for your gauges.
I opted to open up the wire harness and strip out everything that I would no longer need. I relocated some relays and shortened some wiring.
I think you'll need a resistor wired in to the alternator for the "exciter" wire to let the alternator know to charge, since the PCM won't be doing it anymore.
The fuel pressure regulator is a Mallory 4309
You will need a different distributor since that is run through the PCM, which won't be there anymore. This will also involve more wiring if you want a functional tach.
New throttle cable and kickdown cable (if automatic)
I'm sure there's a lot I'm forgetting yet, but basically it's not "slap a carb and intake on and go".

 

What carb and intake do you want to swap? Factory CC carb of the holly carb with edelbrock intake?