I towed 10K without issues with my 64 GMC 3/4 ton.
Best to look into a 70's chevy C20 or C30 with air bags. I have that setup on my 77 chevy C20 and I can haul whatever you can throw in it. But you want 66 or older, I'd look into a 64-66 C30 which will more than likely have a flat bed. Also look into a set of air bags, you'll be Much happier with 5K in the bed. One ton leafs will do OK, but will still squat like crazy with that amount of weight.