Hello,
I'm running virtually the exact same setup as you. I have an 8.25" rear with 3.21 gears but used to have a 7.25" with 2.76s for a long time. There is definitely a very noticeable change with the gear swap. 3.21s are much more fun that 2.76, yet still allow highway driving. I'm running a Hughes 2500 stall that I got back when I had the 2.76 gears. I remember before, the car would kinda shimmy when I stabbed it from a stop because of my cam and the 4 bbls of carburetion wanting in to the motor. I think the stock stall speed was 1500 rpm so I made a big change for a street car. The converter helped alot. Now if I punch it from a stop it'll stand up, spin both tires for a sec (traction bars) and GO, whereas before it might squeek the tire a little bit and get going ok. Plus, when I punch the gas, obviously my motor instantly jumps to about 2500 rpm (hence 2500 stall), which means my ignition advance will be 100% fully in the second I punch it from a stop which helps even more. Also, with any cam more than .420" lift and .204 duration @ .050" or around 270 adv duration, more converter will help. My transmission class teacher thinks 2500 is a bit much for my motor but it works great. The big drawback is it knocked my gas mileage down about 3 mpg. so the answer to your question is yes, the converter will help get you off the line much quicker, and the 3.21 gears will also be much more fun than the 2.76s. However, If I had to do it all over again, I would go with a 2200 stall instead of a 2500 so that's what I reccommend to you.
Good Luck!
