What do you have to do exactly to convert a 13b turbo manifold to a 12a?

 

Exhaust? Yoo could make a custom adapter plate, which might cause issues with room between the turbo and the framerail, or somehow cut a section like 20mm lets say (don't know exact number) out of the manifold between the two opening and weld the cast iron back together. I think.

If you had the tools and knowledge to do that you'd probably be better of making a custom tubular turbo manifold.

 

Dig up my build thread

 

I'll slap some pics up from your thread