So as I understand it, my 66 poly would be a 318a engine. I currently have a 4 speed behind it (original I think) with a granny low and a 3:91 rear end. Where I live, freeway speeds are 75 and most people run 80 or more. This truck as currently configured cannot reach those speeds, that is why I am looking at an automatic with overdrive option. Has anyone done this conversion before and have a list or parts and instructions available? I am looking at using an a518 46RH trans, but I cannot get a clear answer as to whether or not this would bolt up behind the 318 poly. Any information and/or guidance would be much appreciated!