Issues

I have a 67 dart gt that I got in August. I put a 318, 904 in from a 68 coronet. I got it running (with issues) but it does run. Took it for its road test today and....when I got to about 20mph it kicked out of gear (like pushing the clutch on a manual). Then it went back in to gear. Sped engine to 3200 rpms and it wouldn't shift.
So this is my first mopar and how exactly does it shift? I don't have kickdown linkage hooked up. On my Pontiacs, there's a vacuum Module that shifts the car. Any idea what I need to do here? Thanks