I've got a substantial crack and all opinions are to replace rather than weld.
The part number is EDB-8431-A and I just can't seem to find a used one.   Any chance the EDB-9431-A would work?   (I did come across one of those)   Or some other number?

Hi there
I'm guessing that casting number is really EDB-9431-A    I believe 9431 means LH exhaust manifold, 9430 means RH exhaust manifold.
http://www.y-block.info/casting/exhaust.html
There are one or two on e-pay but check carefully as some are cracked and/or welded. 
Depending on where the crack is I would grind out with a burr and braze using a pre heat and a slow cool.
I would think someone on the forum might be able to help.
Why are there lots of  9431  but never  any  9430?
stuey

I might have a EDB- 9431-  A  manifold,i will check what I have and let you know  Carl

Thanks Carl, let me know.

Thanks Stuey!   I hope you're right about the part number, as that would make my life way easier.   

I checked my 55 Merc 292 today and it has ECH- 9431- A exhaust manifold,i am sure it is original.The EDB- 9431- A  manifold which is probably a 56 will work for you though  Carl

thanks for checking!~