The stock u-nuts are M5. The hole center to edge is 13.5 mm (or close to it). The bolts have an 8mm head if I recall correctly.
I changed things on my Jeep. Since the rest of the bolts for the upper radiator support require a 10 mm wrench/socket I went ahead and switched out the u-nuts to M6-1.0 with a 13.5 mm hole center to edge. I got the nuts off ebay. The ones I got are made by Au-ve-co products, P/n 12669. They also list part numbers 1016937, 11503957, N623332-S, I think as GM & Ford cross-reference numbers. You can probably just search for "Extruded U-nuts M6-1.0" and then look for the right hole center to edge measurement.