Pretty much anybody selling authentic reproduction Chrysler fasteners is buying them from AMKproducts.com, because AMK is pretty much the sole source. You can buy from AMK too. Little Detroit is not listed as a dealer, but it's possible if they're buying from AMK in large quantities that they are re-selling cheaper than AMK for small quantities.

Apparently AMK does not have a "master kit" for body bolts, but they definitely do sell all the correct panel fasteners you'd need, meaning you need to have a bill of materials and buy to that, fastener by fastener as listed in AMK Bulk Fasteners. Maybe Little Detroit has made a bill of materials and offers a kit.

For A-body (sorry, I don't have a B-body) definitely all fender fasteners are grade 5 machine screws with captive convex washers, 5/16" diameter, with U-nuts, all phosphated, and the machine screws have makers' marks like "SEMS", "GL", and anchor. Grille panels and brackets behind the grille are the same, except 1/4". The only zinc plated fasteners are a few shim washers and the hood height adjusters.

I suspect A-body fender bolts are much the same as B-body, except maybe 3/8" instead of 5/16". FWIW here's a 1969 Barracuda bill of materials and assembly sketch for fender fasteners that might help somebody.