The biggest engine that can be relatively easily installed with minimal modifications would likely be a Ford Windsor 351 cubic inch (5.8L) V8. This engine is a popular swap choice due to its relatively compact size compared to larger engines, and it utilizes many readily available parts. However, even with a 351, you'll likely need custom motor mounts and possibly some chassis modifications.
Larger engines like the 460 big-block are possible, but require extensive fabrication, including custom motor mounts, transmission crossmembers, potentially modifications to the firewall and inner fenders, and possibly even frame reinforcement. It becomes a far more involved and expensive project.
Therefore, while a 460 *could* theoretically be installed, a 351 Windsor is a more practical limit for a relatively straightforward swap. Anything larger pushes into serious custom fabrication territory.
