A little late to the party but here's what I've learned over the years regarding the 6.4 BGE trucks.
Torque converter is terrible in these trucks. Fortunately you can use any 545/65 converter on the market. I'm using an Edge now which was a huge improvement over stock. You can also use a MagHytec oil pan for additional capacity and cooling. As mentioned, many of the internals are straight out of the 68RFE so likely you won't blow up easily with an NA build.
Headers were a huge improvement on my truck, dropping a second 0-60 even before I tuned it. Hemifever has good tunes if you go the Diablosport route, especially with the trans tuning. Along with the Magnaflow catback it gives the truck a very aggressive sound with minimal drone. I also added a ported TB and modified air intake, but those were very minor improvements.
There is really no reason to start with the block or heads since as mentioned it's actually an upgrade over the SRT stuff. Compression is a point lower, so that hurts, but the bigger issue (according to Modern Muscle anyway) is the camshaft. I've been debating an upgrading, but was also considering a blower instead.
Speaking of, Whipple is supposed to have a 6.4 truck kit available any time now, or you can go with a Procharger kit which is available today. Issue here is whether or not the tranny will live. From what I've read at much over 500hp even the 68RFE (stock) is on borrowed time. With a good tune (and maybe exhaust) any 6.4 blower kit should easily exceed that. The build Arrington did with a Magnacharger was a one-off and they wouldn't even provide details on that build. Biggest issue with adapting a car kit is position of the TB as mentioned, and the elaborate EGR setup which is tucked in the valley under the stock intake.
