What do I need

I am going to make my 1955 Bel Air a performance vehicle, this is what it has right now

straight 6 engine, upgrading to a performance 350
3-speed column shift trans

And everything else is stock for this set up, I figured I would have to beef up the rear end, put a different trans in, hoping to find a 3 speed out of a truck I can use. But what kind of things will I need to replace or upgrade to have a great performance running Bel Air? Like cooling, suspension, drivetrain, etc. Thanks for the advice.

Since you are upgrading, what is the condition of your original three-speed and are you willing to part with it?

Why not go to a Mucie or T-10 4 speed, or maybe a Tremec with 5 or 6 gears??? If you're going to make it a driver, might want to go to an overdrive type of trans. You can run 4:11 or so rear end ratio, then with an od in 5th gear can cruise down the highway at a reasonable rpm....

Might want to include a front disc brake conversion, too. Got to be able to haul the beast down safely, too.

Find a buildable 350, get a 383 stroker kit and some good heads. Lots of low rpm torque works great on the street.

As for the differential, use a 9" Ford or a 12 bolt chevy, they are reasonable priced and will stand up to reasonable horsepower applications without a lot of high dollar mods.