When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The factory tire in 1966 was probably something like an 8.15 x 15 2 ply, 4 ply rating car tire. (or maybe 8.45s) Almost everything in those days was undersize tire wise.
225 70 x 15 or 235 70 x 15 or similar would be a decent substitute. You will also find these in 75 aspect ratios. And in LT load ratings too, methinks.
You can check tire diameter vs. metric size here, 4th calculator down:
I would go out on a limb here and say Ford never offered a 15x8 or 15x10 on any truck, and probably never on any car either, to this day. (15x7 into the '90s then the diameters went up to 16 & 17 & beyond.