A 261 would be a direct bolt-in, and can be made to produce good power (there should be an article about them in the tech section). A 302 is a bit more work to install, but would also provide good power. A 292 would probably give you the best power and be a higher-rev engine, but would be by far the most work to install. With the rear axle gearing and brakes in a 6400, you're not going to be moving very fast no matter which engine is installed.
