I would take a really good look at the sprag assemblies. What you are describing sounds very similar to what my 65 did when I broke the input sprag. Sometimes it would grab and you could drive slowly, but once you put a load on it, it would fail again, and basically be in nuetral. The last one popped a few times, I could brake boost it a bit, but on launch it would release and the car would not move. Eventually it failed completely and I had no first gear.