Beleive it or not, the only exact bolt in frame for your 66 without doing any mods is a 66.
65 is similar but brake lines are routed differently, easy to fix by drilling new holes for brake line clips.
67 is similar, but would require removal of 67 emergency brake bracket and instalation of a 66 emergency brake bracket.
63-64 both have different front crosmembers, line routings and additional frame mounts
New frames are available and IMO are nice, the welds are modern mig and lack the drag marks of an arc welder, but for a driver car you really cant beat them. by the time you find a good frame, adapt to your year, have it cleaned and painted (and have to fix anything unseen at purchase like "its bent" terrible to find out when its time for a body drop.) probably around the same money.
You can take a later frame 68-72, change the body mounts, add emegency brake parts, change the last few feet of the frame, and modify the front frame horns, and make it work, but it can always be seen as an incorrect frame without many hours of labor to make it appear orig.
Dennis