When using the MP aluminum Stage VI heads on a 413-440 you need the spacers. On a 383-400 you use an RB manifold.
I'm not sure that is the same thing...
I’m just going to add this in case it might help someone in the future......
The stage 6 spacers can probably be made to work to install a B intake on an RB block, but will likely need to be modified some.
The stage 6 heads have raised ports, the stock heads do not.
I haven’t placed a stage 6 spacer onto a stock head, but I think the lower edge of the space would hang down below the lower edge of the head.
That extra material would need to be removed from the spacer so the stock valley pan(or other valley cover) would fit between the spacer and the head.
Even when used on stage 6 heads, these stage 6 spacers do create a little dog leg in the flow path.
These spacers were the difference between B and RB manifolds. Years ago I used a set to install a B manifold onto a 440.