If you plan on use plain m12 bolts you may need to sleeve the bolts to allow the harness brackets to roatate freely unless you plan on using eyes. My harnesses came complete with the 7/16 bolts and the sleeves.

The IVA tester will look for movement in the harness end, usually provided by the tophat washer and the wavy spring washer provide this.
Eye bolts and seatbelt clips are also acceptable and are generally metric coarse thread.

Just thought I'd mention, my IVA tester said m12 bolts are fine for seatbelts. So long as the length it ok.

He also mentioned, the roll bar doesn't have to be threaded, you can have the bolt going through a sleeve in the roll bar with a nyloc nut on the bottom.

I've used M12 on my Locost simply because it was free to me if I did it this way.
The one issue I have now is that the harness brackets don't rotate on the bolts and are captive and I had to widen the 7/16" holes in the brackets ever so slightly. None of this was an issue at SVA. The tester wasn't interested in the fact the belts were captive. However, I am and am looking for some way round it now without redoing the whole thing.

I wouldn't alter the hole sizes on the belts, if they are supplied to suit 7/16 bolts then use 7/16 bolts...It might come back and bite yer arse if you ever needed to claim on insurance. Belts are a "safety thing" after all.

It might but then again it might not. I doubt they'd notice as it's only around 1mm bigger in the diameter, so 0.5mm off each side.