Thanks, that's interesting. I guess there could still be a double spline on the stock arm and FF's fast ratio arm just puts two splines into it.
But I'm sure the pitman arm position will make a difference to where the steering wants to center because of way the Ackermann steering geometry works.
If the arm is centered correctly and with a small amount of toe-in when the wheels are straight ahead, then turning the wheels either way takes them into a toe-out alignment that they need to take corners correctly (because the wheel on the inside of the corner is tracking around a smaller circle).
If the arm is not centered correctly then turning one way takes the wheels into toe-out whilst turning the other way takes them momentarily into a deeper toe-in before they go toe-out. The wheels try to go relatively parallel with each other and it causes them to pull to one side to neutralise the imbalance.
I'm not sure whether the pitman and idler arms are supposed to be exactly parallel, but if they are then they will still be parallel with the arm fitted wrong because the distance between either end of them can't change. If they are not supposed to be parallel then the straight ahead relationship will be different with the arm fitted wrongly, but it might not be enough to see.
My tyre pressures are equal; in fact everything else seems to be equal. I think I'd better start finding a puller for the arm.