If a 4" x 6" will work I can get a set on Amazon pretty cheap.
They won't.
These Genssi H4701/H4703 are the only option I'm aware of that are actually a proper LED housing and will fit the quad lamps in these trucks. H4701/H4703 are smaller than 4"x6".
https://genssi.com/sealed-beam-led-replacement-headlights-for-gmc-c1500-2500-3500-trucks-2-pack/
Everything else linked in this thread so far looks to be a reflector housing with the drop-in LED module which will undoubtedly perform worse than the Genssis, or the wrong size.
I'm surprised they're claiming DOT approval, you wouldn't think going through the process would be feasible for niche items like this. Makes me wonder if it's legit or just more China stuff making baseless claims.
I would lean towards the latter, but unfortunately I don't know of any US based companies (or any reputable big names really) making LED housings in this size.