Does anyone know what the difference is?
I'm not talking about stock, but if you were buying a sidedraft manifold.

I bought a manifold that the seller told me was a 13B manifold. A 4-port. When I got a 12A manifold for my car, they looked exactly the same.

 

I bet the 13B manifold is wider because of the wider rotor housings. Check where the holes in the manifolds would hit.

~T.J.

 

Even if you're not talking about stock manifolds, this link will show the differences as the block sees them and should provide a clue as to why it won't work...

Intake Manifold Differences

Oops...my bad...the link concerns only 13B manifolds....

 

That's what I thought, because I put both the 12A manifold and the 13B one together and they are the same size. Maybe this seller guy was wrong.

 

Sounds like your seller may not have known what he was talking about. BTW, there is an adapter to mate a 12A manifold to a 13B block, I think Racing Bleat sells 'em, or at least they used to. I have a friend who used one on his 13b converted RX3 wagon and it worked well for him.

 

How much does that adapter plate cost? I was figuring I could just have one made from some steel at a local machine shop. Just use the gaskets as a template.

 

I'm not sure what they cost, I'm pretty sure they were aluminum, though. It wouldn't be hard to have one made by any competent machinist. It does add a little length to the runners, which is worth consideration, as that will affect the power curve somewhat.