Floating shelves with threaded rods

I'm looking to install 3 80" 2x12 reclaimed maple planks as floating shelves in my kitchen. My thought is to screw 12" threaded rods 3+" deep into studs and slide the planks on through bored holes.

The planks currently weigh about 25 lbs but with the load I'd expect it may reach 75 lbs total.

I was thinking of using 5/8 rods but it's tricky to correctly bore these in the thin wood. I'd like to go with 5/16" rods. Is this too small? What is the minimum width to handle 75 lbs over 5 studs?

Thanks!