how many old mag race boats are still out there,

Terry the manifold I posted here does not look like yours but would work with your riser. Yours looks like Edelbrock or Nickson but they have not been made in some time. Check the centers of your engines most likely yours are on 30" centers, finding manifolds to fit may be tuff.
Don't know for sure how many Magnum race boats still around but I know this Lake St Clair one the old Charge It of Bob Kehrig now known as HOT NUTS is no longer as it sank off Naples Italy & is on the bottom. Iron Head here on OSO has the Anaconda & Subluxator Lake St Clair race Mags in his collection.

http://www.cpperformance.com/p-12887...ock-chevy.aspx