Interesting thread. What exactly is your design goal? Are you talking about a simple "meeper-style" caliber conversion? OR are you talking about a fully-tricked-out "loosely based on an AK" custom one-off build?
I would consider a meeper-style conversion more than a "custom one-off" and build it based on the AK-100 style rifle. Galil? Well, there's a lot of good reasons to prefer a Galil but I think I would lean towards the AK-100 tweaks. Think AK-74 with updates. This is more of a personal preference I think as both designs have their good points, and the Galil has a ton of "extras" that would be more work to add to an AK design - night sights, bent charging handle, different rear sight/top cover concept. That said, you COULD add these things to an AK with some work. Night sights being the most work - maybe strip them from a beater Yugo? Not a big fan of the milled receiver - I understand why they did that, and don't see the need for the extra weight. Again, personal preference.
THAT SAID... let's look ahead 6 months... start with your 6.5 Grendel barrel from any of the custom AR barrel builders. Lathe it down to the proper journal sizes, sleeve the breech if necessary (I have no idea what the chamber diameter is for an AR... need to dig in my safe). OK, you've got you're barrel now. Next, you get a "chopped" AK-74 kit, press off the barrel components, press them onto the AR barrel, and drill for the handguard retainer, barrel pin and cross-pins like any other AK. Put on your choice of receiver and you've got your rifle. It would look almost identical to an AK-74 or AK-101. Put on a (folding) stock of your choice.
This leads to my only other consideration - magazines. What kind of magazines will properly feed this caliber. I STRONGLY SUSPECT that an un-modified AK-74 mag will work, but I need to surf around for more pictures of the two calibers before I'm convinced. If not, choose standard 5.56x45 caliber mags (like Galil mags) or AK-74 mags with Robarm followers.
"Done". Trick it out with whatever scope/tri-rail/adapters you care to buy.
Again, that's all for a "standard caliber conversion". I've given thoughts to this in the past few weeks due to the ominous threat of barrel supply issues we face. I'm pretty sure all of this will be doable, and there may be some minor issues to contend with, such as extractor problems with an AK-74 bolt. No idea on that, need to research cartridge dimensions more.