Should'nt be an issue if you're using all factory stuff. Are you sure you have the shift arms installed properly? Post up a picture so we can see what you are dealing with.
can you post a pic.? im sure someone can get you fixed up.Did you get this resolved? I have the same issue on the 65 that I bought a couple years back...can't get it into reverse. I haven't done the research to figure out which of the linkage or tranny isn't original.
Pics are below. I think the tranny's a Saginaw based on the 6261884 casting number on the tail. The body's casting is 8923(or 5...can't tell)856 which I can't find anything on.can you post a pic.? im sure someone can get you fixed up.