Not something I've tried, but the early A body engine compartment not only is significantly narrower than 67-later, but the early K frame and steering gear box are two inches further to the rear than the later ones. Can't see a shorty header designed for a 67-up A body can possibly fit in a 65 Valiant.
If ground clearance is your only concern, TTI and Doug's both make long tube headers for an early A. Not cheap, and not so easy to install, but they do fit, and have good ground clearance (nothing below the tie rods).
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