Headers suck...they leak and they rust out and they burn up the plug wires. Unless it is a serious race car, go with the best set of stock manifolds you can get. they do flow really well out of the box. My buddy built a 400 (upped compression, Comp Cams camshaft etc) for his 78 NYB and I found it interesting that he used stock intake and exhaust manifolds (he used 75-78 HP exhaust). I'd say he was right because his 400 was a downright angry engine that spun the sure grip at will.
Most 400 ci 1978 NYB's came with 8.25 axle. What does your buddy have in his?