The earlier frames seem to rust less then later models, but in Cali that may not as much of an issue as other places. I would personally combine the two and make one good one. If you go with the I6 motor the 76 is a 2F and a better motor that can be desmogged in the '66 frame. The 76 would have disk brakes and birfields on the front and fine spline rear axles. The 66 axles make good boat achors.... so I'm told. The 4 speed in the 66 may be a sm420 or sm465, either would be good with the early (3 speed) t-case in the 66. The 76 t-case (4 speed) doesn't have as low a gear as the 3 speed. Combine 'em, sell the extra parts and have fun.
Just my .02