Seachaser,
Wait. I just had a thought. What year is this boat?
Do you have a 6.354, e.g. 6.3541, 6.3542, or 6.3543...
...Or do you have a "Range 4" "TW" series 6.3544?
Does your part look like this manicooler?
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Perkins Manicooler Replacement
I ask because I don't happen to know of any truly weird manifold on the Series 1 - Series 3 6.354 engines.
But, if you've got a "TW" series T6.3544(M), you've got a very different manifold. It is freshwater cooled vs raw water cooled.
I think both Mesa (the place in Arizona) and Greenwater marine do the elbows. Here is Greenwater Marine's info:
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https://greenwatermarine.com/
I think a bunch of others do the end caps. And, usually, it is just the elbows and endcaps that fail. Maybe you can do just those?
As for the manifold itself....it is, actually, a "little bit" of a unicorn. If S&W can get you one for $2500 -- I think I'd call Mesa to double check the price, and then take S&W's deal in a heartbeat. The rumor I heard is that the original manufacturer (Serck) in England was bought out, merged, re-formed, and hsn't had anything to do with that business line in a long time and that there was never another production supplier.
That water-cooled manifold isn't really a unicorn in that some folks know how to make them and will do it on an as-ordered basis. But, I think that is typically a ~$4-5000 deal. So, it isn't unavailable, or hard to get, just expensive.
But, it is "a bit" of a unicorn in that if Mesa doesn't have one on a shelf, I doubt anyone does. And, last time I asked -- they did not (but they were planning to make a few).
My thinking is that if S&W can do it for $2500, Mesa may have done a run and have a few on the shelf. And, if not, it'll be more expensive for them to make one for you -- and a few more for the shelf.
But, again, without the numbers, I am just guessing here. I'm trying to figure out in my head why this is hard in your case, but so much easier in so many other cases.
The numbers would tell me what you have (a picture might, too)
If that's the case and you can get one for $2500-$2800, that might be a deal. Thoe were made by Serck in England.