Dakota manifolds on LA J heads?

[Demonic]

This is for my 360 in my 75 D100 truck. The existing manifolds are the ones with a common center port - no divider. I’m thinking the later manifolds flow a bit better than these center dump truck ones. Are the ports close to J head shape? Anyone done this with a truck?

 

[Professor Fate]

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.

 

[NoCar340]

This is something into which I looked quite a few years ago. Memory's a bit fuzzy and I may be wrong here, but as I recall the exhaust manifolds changed in 1994 for the T300 Ram chassis. The newer design supposedly doesn't play well with the older trucks. The bright side is that the older manifolds have a larger output (2.25" v. 2.00"). I don't think Dakotas used different manifolds than the full-size trucks, but they changed at the same time.

By no means is this gospel--absolutely double-check me. I do know choking the output size down to 2" allowed them to later utilize "natural" exhaust-gas recirculation (reversion, really) and eliminate the EGR valve.