Coolant tank/sensor problem

[pjblakey]

I just replaced the coolant tank in my 1999 with an aftermarket model tank. Not too bad if you take your time.
I reinstalled the level sensor, topped up, bled the system and got rid of the air pockets.
Trunk is now dry and no overflow.
However, the coolant level sensor flashes continuously, and I think I have found the cause but don't know how to fix it. Maybe someone here can help.
The sensor checks good. The float slides up and down a spindle, rather like those in some toilets, and in the aftermarket tank, the spindle is 2" shorter than in the original tank, hence always activating the sensor. I determined the top of spindle position by measuring down the coolant opening to the top of the spindle. The float is free to move, but is 2" lower than in the original tank.
Are there different tanks and/or sensors for 1999 Boxster?
Or is this just one that has been manufactured incorrectly?
Original tank p/n 996-106-147-04. Replacement 996-106-147-08. I have been told they are the same tank.
I'd rather not pull the new tank if I could install another, different sensor.
Any relevant info will be appreciated.
Peter