New receivers are being manufactured in the US, without the holes where the selective-fire parts would pin in. Take one of these, with its US-originated serial number, and substitute an arbitrary number of other US-made parts, and it's legal per the Assault Weapon Ban (that will, if we mind our Congress, sunset in just a few months). However, these receivers are steel and might be rather heavy.
Let's get somebody to make an alloy or carbon-fiber Galil receiver. Let's get somebody else to make a carbon fiber-wrapped 6.5mm Grendel barrel at about 18" length. That keeps the weapon's weight down to the range that a Grendel deserves, 'cuz the original Galil (milled steel receiver) was on the heavy side. The carbon barrel should also be immune to point-of-aim shifts as it heats up.
Open up the bolt face and reface the extractor so they take the Grendel's case head.
You might have to tweak the gas system. Emphasize might: the Grendel's developer claims that pressures were very close to those of 5.56mm military cartridges, so a gas system that's reliable with 5.56mm should need little tuning to make it hum with 6.5.
Replace the gas tube with a new one with Weaver rails on top. The front-mounted rails let you mount the modern gizmodified intermediate eye relief optics.
There's WUTT!'s competitor to the AR15 platform, in a damned rugged form factor, firing the most capable cartridge that will fit in that form factor. Fixed and folding stocks available. Tritium sights standard.