Generally speaking, a smaller Vortec (like a 4.3L/262ci V6 or perhaps even an early 5.0L/305ci V8) might *fit* with significant modifications. This would likely include custom motor mounts, potentially modifying the firewall or inner fenders, and addressing the exhaust system. You'll almost certainly need a new wiring harness and computer.
Larger Vortecs (like the 5.7L/350ci and 7.4L/454ci V8s) are highly unlikely to fit without major, extensive modifications, and potentially significant fabrication. The engine bay in an '81 Chevy pickup is relatively small.
In short: While *physically possible* with some Vortecs and extensive modification, it's not a straightforward swap. It's a project best suited for someone with significant mechanical experience. You'll need to research the specific Vortec engine you're considering and compare its dimensions to the engine bay of your '81 Chevy truck. Online forums dedicated to classic Chevy trucks are excellent resources for finding out what others have done and the challenges they faced.
