[LordNatedawg]

You probably want to stick with the 245s. I have those on my 1966 1500 as well. They're a fairly common size used on the late 90s to early 00s GM trucks. Easy to get, and fill the fenders nicely.

As for load range, that entirely depends on what you plan on using the truck for. Are you gonna use it as a grocery getter? A weekend cruiser? Using it as a truck? If these are just scrap tires just for rolling around, get whatever you can get.

If you plan to treat it like the 3/4 ton it is, go with LT tires. If it's just gonna be a cruiser and occasional truck usage, then P tires are perfectly fine. I personally plan to use LT tires on mine simply because my truck is gonna be used and abused, just as GMC intended.

I use P tires on my 2012 GMC 1/2ton (standard load/4-ply) and that truck sees some towing and hauling, but is mostly a daily driver. Both the 2012 and 1966 weigh roughly the same, and have similar GVW ratings. So tire rating entirely depends on planned usage.