Some progress....
The left side manifold as used on "our" 318 is most definitely from a 1962 Plymouth Fury Canadian model. Just had a look at one and measured it and took pictures.
Don't know if the US version is the same and if other models are the same, but at least we have one definite source.

Anne

Managed to buy a nice Dodge Polara left side exhaust manifold, same as used on our cars. This was from the USA so there is no appreciable difference between Canadian and US. They call it a B body car...whatever that means??
$20.....the sort of price I like!

Anne

Well done Anne, this is really useful information for the longer term for all 407 to 410 owners. The poly 318 is worth the trouble it is a great engine !