I know it's a lot of trouble, but if you are REALLY interested in trying to get over 2 HP per cubic inch out of a slant six, I would think that you ust would HAVE to build a dedicated header and not try to use the stock exhaust manifold at all.
My header is just my own interpretation of something that might work efficiently. i HAVE no idea WHETHER IT ACTUALLY DOES.... Here are a few pix...
