* Body style: A fastback has more room than a coupe, which has more room than a convertible.
* Modifications: Are you willing to significantly modify the chassis, firewall, and/or engine bay? Extensive fabrication opens up possibilities.
* Transmission: The transmission and its associated components will take up space and impact what fits.
* Axle: The rear axle's capacity and strength need to match the engine's power.
* Suspension: You might need upgraded suspension components to handle the increased power and weight of a larger engine.
With *significant* modification, you could potentially fit incredibly large engines, even some exceeding 800 cubic inches. However, this would be a major undertaking, requiring extensive fabrication and potentially weakening the car's structural integrity.
Without significant modifications, you're looking at engines within the range of what was commonly found in larger muscle cars of the era, perhaps up to a 460 cubic inch big-block Ford, though fitting even that would likely require some adjustments. Anything beyond that generally needs considerable custom work.
In short: "Largest" is relative. A realistic upper limit without major fabrication would be in the 450-500 cubic inch range. Beyond that, the sky's the limit, but it's a *very* different project.
