You did not state your goals for the cars performance. Many folks simply want 'better' acceleration.
'Amping' up a 327 (cam, heads, carb, exhaust, as Dave detailed) to wind to higher rpms to gain hp (in those higher rpm ranges) and mating it to the powerglide (likely mated to a 3.08 rear) will not do much except empty your wallet.
Now, put Dave's engine in a Nova (much lighter than an Impala) with 4.11 gears and a close ratio M22 4 speed, and you're ready to race!!
The +400 lb-ft of low end torque the '383' produces is a more productive solution to the 'better acceleration' quest for the 'heavy' car. That torque will 'pull' well, even with the powerglide/3.08 rear. Back in '66, one had to get a big block to get that torque, today the '383' small block does it easily.
Simply adding dual exhaust and the 2 1/2" manifolds will help the engine breathe/accelerate 'better'. And, if it's done the way GM did it, the car was built for those changes and everything FITS. And, if you're not satisfied with that much 'better', those changes support the 383.
Suggest you compare the cost of buying a crate 383, to having your 327 rebuilt into one, if you are so inclined.
In my opinion, if you are only going to 'cruise', save your money and stay with what you have. Spend it on something else the car needs to drive it. But it is your money and your choice. Good luck.