You can try Auto Air and Accessories. It's in Blaine close to the National Sports Center. I know they don't do additions/conversions, but servicing existing systems for old dogs like ours might still be something they do. Obviously now most of that they do is far more modern.
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Bryan-NW 'burbs 1972 Malibu Vaguely stock appearing, and the opposite of restored. 1999 std bore 5.7, Vortec heads, Holley Stealth Ram, GM cam 700R4, Viking coilovers, 12 bolt 4.10 posi, and a whole bunch more