are you pushing the crank back and forth, make sure the thrust is right. Some times the crank isn't sitting in the right place, its to much on the sides. Where the bearing journal meets the throw. I hope you understand. 4-5 machine shops that i have seen will take a brass mallet and whach it a few times to get the right thrust clearance to check.
My 406 did that, wouldn't turn when assembling it and then my friend came over and whacked a few times back and forth to make sure it was sitting correctly inside the journals. makes sense?