CMI makes elbow top headers to fit. They show a list price of $3100 for satin finish (plus tailpipes). I did this, along with through-hull exhaust on a 21-footer, and picked up almost three mph. We figured it was good for about 25 hp!

Try www.glenwoodmarine.com
I use to have edelbrock manifolds on my 351W but Edelbrock doesn't make them anymore.
I have a COMPLETE 351W for sale if you know of anyone needing one. Complete from Flame arrestor to oil pan , NEW Holley Carb, New Exhaust Manifolds (Stock), Edelbrock intake, New circulating pump. i pulled it from my Pantera 24 when I sold it. My wife wants it out of the yard like NOW! I'll let it go complete for $1,000 firm, buyer picks up.

P.S. - That is the old carb you see in the pic. The new one is a "fully polished" Holley 650 cfm.