All 1 or 2 bbl cars have single exhaust and all 4, 3x2 and 2x4 bbl cars have dual exhaust, no exceptions on any year.
That Magnaflow system is even advertised 'header back' or 'cross-member back' so it's planned to use with headers.
I just order the same system, i'm planning to use some 2.5" headpipes from 440-6 with 383, but i can't say nothing of small blocks.
-Juha