One thing to remember about these cars: It's not as easy as you think to just plop a SBC in there. Most people don't realize that.
-You WILL need to find a different transmission. The car has a BOP-bolt pattern TH350 now that will NOT bolt up to a SBC.
-You will have to source different motor mounts for the SBC (Camaro ones will work)
-If it's a Trans Am, you have a shaker hood. You'll need to figure out how to make the shaker setup work with the SBC. SBC's did come in some 1981 cars with shakers, but they are made of unobtanium. And whatever you do, don't bolt the shaker to the hood. That's just wrong!
-Fuel lines run down the other side of the chassis with SBC's. Easy enough to change.
-Accessories and brackets are all different. Nothing swaps over. The only interchangeable parts between the Olds and a SBC is the distributor cap and rotor. The coil and module are even different! Ask me how I know... I found out the hard way... You might as well be swapping a 5.0L Ford in there.
This is a lot of work. To swap my Pontiac 400 into my car, we had to change the fuel lines, get new accessory brackets, different distributor, engine motor mount brackets, fuel pump, swap the shaker, and I think that's it. Just about everything else fit fine. I believe the 400 is even lighter than the 403 too. They also make aluminum heads and all that stuff for the Pontiacs to make them pretty light. It was still a lot of work, and I'm still ironing it all out, but it was much easier than putting a SBC in the car. And if you go this route, please do yourself a favor and get some nice headers or Ram Air Restorations reproduction long-branch manifolds. I made the mistake of getting cheap Summit 4-tube headers, and they hit everything and killed my starter. It's still sitting dead in the driveway.
If you want to go through the nonsense to put a SBC in there, just get a LS-series motor. For the money, it can't be beat. You can usually find a parts F-body with the T56 for a reasonable price, snag all you want from it, and then scrap the rest to recoup some funds. And besides, if you want the car to handle better, ditching some weight up front will help balance these pigs.