Dakota manifolds on LA J heads?

I take it you're referring to Magnum manifolds?
Some people swear by them because they do flow well; but if you look at them closely you'll see the port size and shape is closer to 318 dimensions and will block a fair amount of the J head's exhaust port, especially on the two end ports.
They fit and will work, but I'm not sure you'll see the "vast improvement" that some people expect when you use them on the early J and X heads.
Believe it or not, later 308/594 type 360 heads actually have a smaller exhaust port window than early heads, similarly sized to 318 heads.