This image is for a 99 5.7L (would apply to other years, but not certain of the cut off years).
Diagram of OEM RADIATOR MOUNTING & RELATED PARTS. Contains 8 parts priced between $3.51 and $87.50. Fits 1999 Chevrolet K2500.
www.wholesalegmpartsonline.com
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The hardware in question is #7 and #10. #7 are the bolts; #10 are the U-nuts. As you can see, #7 and #10 apply to the top and bottom radiator shrouds.
#7 is GM P/N 15029272 which is an M6-1.0 x 25 mm bolt.
The Wholesale GM Parts Online website skips #10, but you can find it on NemiGAParts.
Parts catalogs. Spare parts catalog Chevrolet EPC Online. Chevrolet K1500 PICKUP 4WD - Carryover Model. Cooling system-grille-oil system. Radiator mounting & related parts.
nemigaparts.com
#10 is GM P/N 11518121 which is a U-nut that takes an M6 x 1.0 pitch bolt, and the U-nut applies to a panel thickness of 0.5-4.0 mm.
The listing on eBay will take care of all that hardware. If you add the listing to your watchlist, the seller will almost certainly send you a reduced price offer.