i have 305 manifolds and i'm going to put on shorty headers soon. i have a flowmaster 80, 3" catback(will be 3" all the way when headers are on), no cat, and 4"x24" tips. its pretty loud but i was wondering how much louder the headers will make it.

 

Not much at all if any. Noise is more of a function of the muffler (and cat, if there).

I did headers/cat/cat-back all at the same time (along with cam and heads), so I don't have a direct comparison on this car. But, when I went from 396 w/factory manifolds to 396 w/headers, hardly any difference.