motor wont turn over?

I can't help but think the plugs are hitting the piston.
I would take all the plugs out and screw one in by hand and turn the motor over by hand, when it stop's like you say, unscrew the plug a couple turns and see if it will turn all the way over.
if the heads was milled down in the past and you don't know and they had a thicker gasket and you put in a normal gasket this could be happening.