8 3/4 gears

Not sure what you mean about changing the "exhaust manifold" because a person don't change just one exhaust manifold generally. Maybe you mean the intake manifold since you mentioned the carb too. That will help quite a bit since the stock carb. is so small. Since your changing from a 7-1/4 to an 8-3/4 rearend it's probably a good idea to get those 3.23 gears. You need as much gear as you can to overcome the weight difference your adding with the 8-3/4. It's alot heavier than the 7-1/4 so the lower gears will help make up for pulling around the extra weight. If you don't already have dual exhaust on your car consider that also. You need to let it breath too. Don't go bigger than 2-1/4 pipes though or you'll loose velocity and actually loose power.