Front bumper bolt size

[MikeWire]

Can anyone give me the size of the front bumper bolts without bumper pads? Do they need to be carriage bolts, or could I use a standard bolt?

I want to bolt on my bumper, but all I have are the bolts from the PO's brush guard, which I will put on later.

 

[MikeWire]

FYI and for the archives, the size I used was a grade 5 carriage bolt - 1.5in long, 3/8 diameter, with 1 nut and 1 lock washer.