I have a 13b with a hitachi carby on it. I want to put a 4 barrel nikki carby on it because the hitachi carby is stuffed. Will the nikki bolt straight or doesn't it fit.

 

You'll have to drill four new holes with a closer spacing. Then thread them with 8mm x 1.25. Then you can move the studs to the new holes. I'm not sure if you'll need to cut channels to connect each primary to its respective secondary runner, like the Nikki manifolds have.

I have a redrilled manifold like this and it doesn't have the channels. I moved the studs back to their Hitachi positions and threw an Hitachi on it. It works great.

If I were you, I'd not cut channels.