Sometimes those dowel pins seize and rust. I always apply a good amount of lubricant on the dowel pins before trying to mate the two.
Be sure to check converter spacing against the flywheel should be 1/8 to 3/16.
Actually before trying to mate the two, check that torque converter is fully engaged. I place a straightedge across the bell housing and measure to the converter pads. I believe it should measure 1"? If that's good then you can proceed to mate the transmission to the engine.