I agree the harness bar needs some means to keep the harnesses from moving laterally. In absence of retaining the lateral position of the harness, you're counting on the holes in the seat's head area to support the harness. Seat holes are only for access, not support of any harnesses.
Ed's suggestion about the muffler clamps is a very good one. If you want something prettier, you can buy some steel or aluminum bar clamps like these shaft collars
McMaster-Carr
Walt,
You're right that the eyebolt isn't going to fail. I was just giving credit for pulling the eye in the best position to carry the load. Also, it seems the clip-in harnesses work best when you've got vertical eyes. Horizontal orientation sometimes causes the clip-in hardware to get hung up in my experience with my own harness installations.
John also makes a good point about the cotter pins in the spring clip. Our race tech inspection guy always checks for those. So a DE tech inspection could possibly check too. May seem like overkill or nitpicky, but I think they're a very good idea for any application. I have had spring clips that were became partly "open" due to the belt loose and allowing the clip door to be aligned with the eyelet. Then when I pulled the belt tight, the clip door was trying to open via my pulling on the belt while in contact with the eyelet, which bent the clip door. With a cotter pin in there, the clip door is not able to move out of the closed position.
If you ever make a version 2.0, i'd suggest adding another piece of flat stock to the right angle "bend" between the floor plate and the eyelet plate. Only reason I say that is because a tech guy could say, I don't know how good your welds are. If you add another piece that provides some additional support, the guy is less inclined to question it because you've got multiple welds that have to fail for the bracket to displace?
You have to consider that your custom mount invites scrutiny because it's not something that was built into the car by the factory, nor is it the tried and true mount-thru-the-floor-with-monster-washers. When tech people see home made stuff, it sometimes raises an eyebrow if it doesn't have a manufacturer's name brand sticker on it? That said I think it's nicely made.
I give you credit for asking for feedback on your install. Lots of people come up with some hokey ideas to connect harnesses. I've seen a few myself on friend's installs. I don't consider myself an expert by any means. It's just that i've seen people be silly because they don't want to drill holes or spend money on an "expensive" store-bought solution. Nice job making your own.
I agree that 3/16" angle stock is PLENTY beefy for this application. Even the factory seat belt locations in the chassis are only threaded bosses placed in somewhat thin sheetmetal. Next time you're under the car, take a look at the rear seat wells and you can see the bosses back there aren't heavily reinforced at all. Granted these are back seats where the Porsche engineers figured little people (weigh less than adults) are likely to be. But the point is, tech inspection people find those points acceptable to connect the harnesses to when you route the harness over the bar instead of looping it around the bar. Some tech people will suggest the harnesses be connected at the rear seat points in lieu of around the bar, because some bars are not well designed to anchor the belts, but only guide them instead.