>Can using a 283 header on a 350 engine alter or affect in any way the vaccum >of the engine?

Probably not to any noticeable degree. Most off the shelf small block Chevy "street" headers are generic with regard to primary tube diameter. When used on a 350 instead of 283 (all else equal), exhaust gas velocity may increase by virtue of the increased displacment, but that ain't a bad thing.

Bob