Lots of conflicting data in here.
Elite sells two versions of a ported manifold. One of which is the heaviest ported manifold available to market. The biggest gains will be seen by going with this manifold, the ZMAX.
The ZMAX jr. is a budget manifold essentially. Having only the center bolt stands milled out. Still offers great gains over stock but at a much more manageable price for most.
Midwest Diesel has three stages of intakes and as well offers the option of upgrading to o-ring boss fittings and bungs in the manifold tops.
Stage 1: center bolt stands are milled out
Stage 2: all bolt stands are milled out
Stage 3: all bolt stands are milled out as well, the factory fuel bowl clearancing done in the casting, is milled out and is TIG welded in with a formed aluminum plate
We sell all of the above. JD runs the ZMAX jr. on his truck and has had nothing but great things to report back in way of gains. I myself have built my own. Been inside one with a mill and know the areas that need addressing and what the best way is to accomplish that. We(Absolute) have stayed out of the 6.4 manifold market simply because the two companies I listed above, have done a fine job delivering a product to market that works and does the job exactly how I would want whatever we would have came up with too. No sense to me in copying what somebody else did and flooding the market with it. Regardless, if you need pricing/availability on any of what Midwest has for intake manifolds, as well as Elite, feel free to let JD or I know. We would be more than happy to help