I bought my bolts at Bolt Depot and I got #4510 - Socket flat head, Stainless steel 18-8, 3/8"-16 x 3". I used them yesterday and they worked great.
Here are a couple of picture of one of the rock rings installed.
and a view down the edge of the tire
I tested my rock rings on 2N17X yesterday and they worked great, although the lack of scratches on the rings at the end of the day showed I didn't give them a hard test. I haven't gotten the inserts installed yet, so I used a liberal amount of anti-seize. The cordless impact wrench really helped with the installation and removal. I found an impact wrench bit at Graingers that fit the bolts perfectly. So far I'm using the box that OMF Performance shipped the rock rings in to haul them around, but eventually I plan to make a custom case using polyethylene foam and rip stop nylon.
Here are a couple of pictures of the full truck.