Hooker Headers on a 69 Mach1 351C 4V (Kenreeves)

I am building a 1969 Mach1 with a 351C 4V engine, it has a automatic transmission, manual steering, aftermarket AC. My builder keeps telling me that the Hooker 6920HKR long tube headers wont fit, but I see countless Mach1's with the same headers with 351 C 4V engines. What could my problem be??