Aside from the headers I would check if the starter clears the chassis. To place a 385 in a "CSX" chassis I had to hammer a huge dent into the rail. Can't remember if I had to modify the footboxes, too.
I guess your chassis clears either block and accessories? Even your fuel pump or
oil filter can touch the chassis and you face more changes.
And what power difference are we talking about? You'll never feel it...