a 66 should have a 6 banger (292 or 250)a 283 or 327 v8...maybe, someone else will have to tell you for sure. No other engines that I know of were used.
The lockout hubs are stock, I have heard but not seen non locking hubs on these. Replacement hubs are available, BUT at about 350 bucks from milemarker.
My 66 does the same thing when revved up, it shouldn't, so it has a charge problem of some sort. Not a big fix, either an alternator or regulator. Simple-ist to put on a one wire alt.
An original motor should have a points ign. Anything newer is better unless you are a resto guy. HEI runs and idles so much better AND everyone has forgotten how to use a dwell meter for setting points. Hope this helps.

64 & 66 GMC long fleetside 4x4
Cartwright, OK