It sounds like you could benefit from some information about the differences in the basic characteristics of the various engine sizes that are potentially options. Chevy V-8s are made in two different basic configurations; small block (262 to 400 cubic inches) and big block (396 to 572 cubic inches). 1968 Chevelles were originally available with both engine types; original options ranged from a 250 HP 327 cubic inch small block, to a 375 HP 396 cubic inch big block. All the big block engines weigh about the same, and all the small block engines weigh about the same. The basic difference between the big block and small block is that the big block is larger and heavier, and has a greater cooling system capacity (to handle the higher power output of the larger engines). The big block heads also have larger ports and valves; and more radical cams. And all those differences combine to reduce the fuel economy and roughen up the running of the big block motors.
What those differences mean in terms of your goals is that a big block engine would typically sound more radical than a small block; and would get significantly worse gas mileage. It would also typically have more low end power; but a well set up small block would not be that much different at high RPMs. Because of these differences, and because you can build or buy a 383 cu in small block that makes more power, is similar in displacement and stroke length to the 396 big block, and is less expensive to buy and uses less fuel than the big block. I would personally go with a 383 small block. Having worked on both small block and big block Chevys for over 40 years, I definitely feel that the small blocks run more consistently, and are more reliable.
But, if you want to go for more power than a small block, I would suggest a 427 big block. 427s have a shorter stroke than the 454s and 502s; and this means the 427 revs more freely, which makes it sound nicer when you get on it. The 427 will also get better fuel economy that the larger motors (assuming you don't put too big a cam in it).
Many people don't appreciate the need to balance and coordinate the internal parts in an engine. Putting a big cam in a motor which has a stock compression ratio will usually make it run WORSE than it previously did. In order to get the advantage of a performance cam; the compression ratio, porting and valve sizes, exhaust system, and intake system all have to be upgraded to a comparable extent. And even after this is done; the modified engine will then have to have the ignition advance curve and fuel mixture recalibrated to match the engine modifications (by running the motor on a dynamometer, experimenting with different combinations of parts and adjustments, and measuring the changes in power). Dyno tuning is the secret of getting outstanding performance (and that great sound); but it can be very time consuming and expensive.
For this reason, I would suggest not guessing about what parts combinations would work best; and instead buying a performance crate engine which has already been researched, tested, and manufactured with proven combinations of parts. A GM Performance parts ZZ383 crate engine produces 425HP at 5,400 RPM and 449 ft-lbs torque at 4,500 RPM. It also has aluminum heads, a roller camshaft, and a 9.7:1 compression ratio, which will run well on pump gas; along with hydraulic valve lifters, which will not need periodic adjustment. This motor is available from Summit Racing (part # NAL-12498772) for $5,159.95 plus shipping. It comes assembled; but does not include an intake manifold, exhaust manifold, distributor, ignition system, or fuel system.
If your budget is not that high; rather than trusting your pet project to the mercy of the local machine shop, I would strongly recommend having a remanufactured engine custom built by the best remanufacturer in the country, at a lower price than you'd pay for comparable work locally. This wonderful company is called Hiperformer Engines, and is in Spokane, Washington. They are the sole supplier of replacement engines for NAPA parts stores, and have the lowest return rate in the industry. Their engines come with a 7 year, 100,000 mile warranty and their prices are a joy to behold. They will custom build a motor to your needs, and ship motors at very affordable prices to anywhere in the U.S.. So by all means, phone the contact number on their website, and speak with their experts directly.