Here's why:
* Size and Placement: The 455 is a significantly larger engine than the options originally offered in the '69 Lemans. Its size and the associated accessories (alternator, power steering pump, etc.) will likely interfere with the original A/C components' location and their plumbing/ductwork. The stock A/C compressor might not even have enough clearance.
* A/C Box Location: The A/C evaporator box is typically located in a tight space within the confines of the firewall and the dash. The larger engine and its accessories could encroach on this space, making it impossible to fit everything.
* Exhaust Manifolds and Headers: The 455's exhaust manifolds or headers might also interfere with the A/C components' placement or their routing.
* Custom Fabrication: While some creative fabrication *might* allow for it, it would require significant custom work, potentially including modifying the A/C components themselves, custom brackets, and potentially even some minor firewall modifications (which you explicitly want to avoid). This is a major undertaking, well beyond a simple engine swap.
In short, while not entirely impossible with extensive custom fabrication, retaining the *factory* A/C system without *any* body modifications is highly improbable. You'd likely be better off looking at a different A/C system or accepting some minor modifications. Even then, professional assistance from an experienced mechanic familiar with classic car A/C systems would be crucial.
