That 403 Olds motor in the Trans Am/Firebird was a real stone. During that vintage, if it had the Olds 403, the sticker on the scoop said "6.6 Liter" and if it had the Pontiac 400, the sticker said "400".
Back in '77, a friend of mine bought a new T/A with the 6.6-liter and it was really unimpressive, especially when passing (stomp the pedal at 45). I mean, he'd be trying to pass some old guy and you'd almost swear that they were laughing at the slow Trans Am that sounded like a vacuum cleaner. I mean, the motor was smooth; it just couldn't snap a rubber band, much less your neck.
In '78, I ordered a new Firebird Formula (back when you could go to the dealer and pick out your options and have the car built for you). I ordered it with the Chevy 350 and a Saginaw 4-speed tranny. Eight weeks later, when the car arrived at the dealer, it had a 403 in it and I wouldn't accept it. The salesman was pissed at me that I wouldn't take the car and the dealer offered to shave the price and stuff, but I insisted that they send the order in again and suggested that they might want to tell the factory to get it right this time. Well, it came and I was very happy with it but I always wondered if I might have made a mistake on not taking the one with the 403 - after all, supposedly there's no substitute for cubit inches. As fate would have it, I was out on the interstate one day and came upon a Formula with the 403 and we got it on. That thing was so pathetic in the acceleration department that I could literally pull away, slow up for him to catch up, and then nail it to pull away again. I was glad I stuck to my guns and didn't take the one with the 403.
With that said, whatever motor you go with, remember this: If you plan on using the original TurboHydramatic 400 tranny that came behind that 403, you are going to have to use a Pontiac, Olds, or Buick engine in it. The bellhousing is different because the P, O, B motors have the starter on the driver's side while the Chevy motors of that genre, big and small block, have the starter on the passenger side. Any of the motors is a bolt-in with the correct mounts since Pontiac, Olds, and Chevy motors were available in that vintage.
My suggestion would be to go with a Pontiac motor since it has great low-end torque and will mate up to the existing tranny. Additionally, you won't have to route wiring and stuff from one side of the car to another liike you would if you used a Chevy motor.
Listen to Maranda. Her daddy's '73 Trans Am would have had a Pontiac motor in it - either a 455 or a Super-Duty 455 (rare). If you switch to a Pontiac motor, you might want to find a parts car of the same vintage for brackets and such that you might need. If you get really frisky and want to go Pontiac, you can do what a friend of mine did back around 1980. We located a 455 H.O. from a '71 Trans Am with a cracked block cheap. The 455 H.O. motor was 4-bolt main caps but so were the 455 motors in station wagons during those years. We got a block from a station wagon at the junkyard, built the motor to the H.O. specs, and put it in a '74 Trans Am. That sucker flied.
Gosh. All this talk about hot rods and horsepower has almost made me feel young again.
Good luck
