400 lbs spring pressure is nothing. Definitely shouldn't bind a gear drive.

Did you verify you had sufficient lash like Rick mentioned in the second post? If you have lash, turn the motor over and it goes away, something is not right. Don't run it.

I never used a PJ gear drive but looking at that dual idler setup I would think if a block has been line bored that would tighten the lash, but I dont know how you would deal with that?

I used Milodon single Idler setups before and you could adjust the lash to compensate. Much better IMO.