Hi Mike you and I need to talk. We have the same boat.
This is a complicated issue but the bottom line is the factory exhaust boxes are trouble. Very bad idea to make the exhaust system literally part of the hull. They stopped it after only a few years due to lots of troubles.
My info comes to me from a Sea Ray technician at their headquarters in Knoxville who has worked for SR since 1975 and actually worked on the line that built our boats.
The exhaust box itself is not structural. These were glassed in after the hull was built. They are basically all just built in place and nonstandard. They invite leakage into the boxed in area outboard of the stringers, and had a back pressure problem that necessitated a factory recall/fix that put a "blow out" panel in, supposedly to avoid engine damage from extremes of back pressure. With the original setup you also run a higher risk of water ingestion into the engines from backing down or at anchor/dockside with waves slapping the hull.
I can show you before and after pictures of what I did which was to glass over the underwater exhaust and run the exhaust thru the transom. I included an internal check valve as an additional protection against ingestion of water.
I have a quieter and safer boat now. It is much quieter than the burbling rumble I had before. I can go over the exhaust parts I used with you.
Once I put some info together for you (assuming you are interested) I can also post to the forum for others.
By the way do you have straight inboards with your 454's or outdrives ?
email me - Joe