Does anyone know for sure what size the lower inner front bumper bolt is? The hex head one on the bottom of the bumper. 3/8" or 5/16"?

GM#3872544
3/8" unc x approx 3/4", 9/16" AF, and would be a "hex flange" I assume as no washer is used with this bolt.. (unless Endura -same bolt but spring locking washer fitted.)
All bumper mounting bolts (excluding some bumper bracket bolts), both front & rear are 3/8" unc.:thumbsup:

Not listed in amk (www.amkproducts.com)'s catalog, but would assume equivalent would be supplied in bumper kit #G-4270 chrome or #G-5099 Endura. $12 each.