Hmmm, interesting bar. No such bar was on my bucket (nor suggested by the dealer). I just noticed the existing holes and THOUGHT BXPanded would take advantage of those holes. I was very wrong, and they had no interest in discussing it with me.
I'm assuming the bolts in your kit are carriage bolts since the bar has square holes. That takes a pretty big punch press to press those out, not something I'd ever even want in my shed. It would have very few uses.
The advantage of the countersunk holes will be that a taper head bolt can be used on the bottom and not make grooves when you're back-dragging in soil. Dunno, maybe I'm just picky, because they wouldn't be deep grooves from just the bolts, however fine roots and stuff will catch on a bolt head and accumulate a packed ball of dirt which will make them deeper. Carriage bolts would be better than standard bolts, but I don't have any square drills.
I kinda like the bolts/nuts with the knurled flanges that don't need washers under them. Not sure I can get them in grade 8, though.
I'll probably use some 1/2" grade 8 taper heads. Just curious if you know what the length is. I can measure it for total penetration plus nut/washer height, but I'm lazy. Don't want threads sticking out of the nuts that will get chewed up and make them murder to take back off later. Now to find a countersink for 1/2" taper head bolts.......