A few months ago, I stumbled across a website of the company above that makes (mostly football) Cards That Never Were. It seems that they specialize in Cowboys, Packers, and Bears sets, but there are many more football team sets from the 1950s to 1970s, as well as a few baseball offerings (such as 1970-style Brewers cards that actually say "BREWERS" instead of "PILOTS").
Here is the site's main link.
These cards look great, and the backs also match the vintage sets (except for a company advertisement).
The only issue I've had so far is that the blue borders on the 1971 set do not match the color of the original set.
Also, they have created a 140-card extension set (the "3rd series") for the 1966 NFL set, which you can either purchase in its entirety, or build your own custom packs (complete with wax-pack wrappers). I have purchased 2 custom packs, 1 of all Eagles, and 1 of all quarterbacks.