it might run better, might not. changing the carb and not tuning for it could make it a lot worse.
on the rebuild kit, without knowing what you actually need $250 is still not a deal. what size are the pistons? what size are yours? what kind of top do they have? what's it going to do to the compression ratio? do they have a stock pin location? will your block or heads need milling? there's literally a 1000 things you want to know before you buy a rebuild kit.
you have a great starting point there. i understand, i spend most of the time i can't work on something thinking about what to work on next.
write down what you want to do to the car, whole thing, however out of reach it might be. blown 408 with a 6 speed, whatever. then do the stuff that needs immediate attention and learn to work on what you have and keep updating the list as you see other cars. a lot of this depends on what kind of driving you want to do with it. cruising around with the family can be way different from a street car you can use for track days. and if your driving the wife and kids around, simple and reliable will keep them happy too
