I don't think the 1977 converter would have worked with the 1966 transmission anyway. I don't know when they switched, but the spline counts around that time were different than the later stuff. I think the flexplate bolt patterns are all the same so another option might be to use the B&M flexplate for external balanced engines with your 1966 converter.
http://www.summitracing.com/parts/BMM-10237/ Other stuff:
You will need to keep the 1977 crank damper due to the external balance. If you swap to the 1966 crank pulley, one of the bolt holes won't line up with the 1977 damper and will have to be enlarged.
The 1977 water pump housing inlet is on the passenger side, so you need to swap your 1966 WP housing on to work with your radiator.
