Practical Limits: While you *could* theoretically fit larger engines, the practical limitations stem from:
* Engine Bay Space: The Ventura's engine bay is relatively compact. Anything significantly larger than what was offered originally requires significant modifications, custom headers, potentially firewall modifications, and possibly even modifying the subframe.
* Transmission: You'd need a transmission that can handle the torque of a larger engine. This often means a TH400 (already a heavy-duty option) or a more modern automatic, or a beefed-up manual. Adapting the transmission to the engine will likely require custom parts or adapters.
* Suspension: Larger engines significantly increase the car's weight and center of gravity, putting extra stress on the suspension and potentially handling. Upgrades might be necessary to prevent handling issues.
* Cost: Significant fabrication and custom work will be expensive.
Potentially Feasible (with significant work):
* Pontiac 455: This is probably the largest engine you could realistically install without *extreme* modifications. Even then, it would likely require custom headers, some firewall massaging, and potentially other adjustments. This was a factory option in larger Pontiacs of the era, so parts compatibility is relatively better.
* Chevrolet Big Blocks (454, etc.): These are also possibilities, but would require even more modifications than the 455 due to differences in engine dimensions and mounting points. This would be a very involved project.
Anything Larger (Highly Unlikely):
Anything beyond a 455 or a similar-sized big block (e.g., a 502 or larger Chevy big block) would be extremely challenging and likely require extensive fabrication and modification, potentially compromising the car's structural integrity.
In short: While technically you could probably squeeze in something larger, a Pontiac 455 is realistically the upper limit for a reasonable build in a '73 Ventura. Anything bigger becomes a heavily customized, expensive, and potentially unreliable project. You'll want to consult with experienced engine swap professionals before undertaking such a project.
