Can a 454 engine fit in 1971 buick skylark?

No, a 454 Big Block Chevy engine will not fit in a 1971 Buick Skylark without significant modifications, and even then it's a very difficult and unlikely project. The Skylark's engine bay is simply too small for the physically larger 454. You'd be talking about major firewall and possibly frame modifications, custom headers, and likely a custom driveshaft. The cost and effort would vastly outweigh the benefit.