Start with 40 grains of Varget behind that Sierra boattail, and work up from there. That will start you out around 2400 fps.
I think the gun builder is blowing smoke up your.....barrel. The chambers for 7.62x51 and .308 are essentially the same. It's the cartridge case that's different, 7.62 milspec cases being thicker and are loaded to lower pressures. Also, loading for a semi is a bit different than for a bolt gun, you can generally go hotter with the bolt gun. I load for a M14 and for a .308 bolt gun. I full size for the semi each time, but I neck size twice before full sizing the .308 brass. I can get more loads per case before seeing signs of impending case failure with the bolt gun, while using heavier powder loads, and I don't have to worry about case feeding issues in the bolt gun like I do with the semi. Did your builder recommend a small base die for the chamber? Some semi's need to be small base-sized so they will reliably feed. I can use up to 43.2 grains of Varget in my bolt gun before starting to e pressure signs, but I don't use Varget with my M14 loads. I'm not sure how the action in an AR10 handles hotter loads, but I have to watch for op rod damage with hot M14 loads. I use H4895 or IMR 4895 with my 7.62 loads
Even if the builder/gunsmith, whatever, says to use only such-and-such, you owe it to yourself to try some different loads. H4895 or IMR4895 works well in the .308/7.62, and 178's have a better BC than 168 grain bullets, so for the longer range they'll usually fly better.
What's the barrel length on your rifle, and what twist?