I have a pair of Eddie Marine EMI Thunder aluminum risers for their center mount manifolds.
These have had about 4" of extra length added to the exhaust tube that can be removed.
These risers take a 4" exhaust hose and the bolt pattern is 4-3/4" by 3".
These fit the EMI Thunder manifolds for the SBC and BBC engines.
They sell new for $640 let alone the cost to weld on the extension.
These were run in fresh water only and look like new with no real corrosion or buildup.
The mounting surface is clean with no corrosion or scratches.
Located in NJ.