Hubie, thanks for the photos. Now we know that your sliding plate has a non standard centre, hence your excessive 5mm gap.
I recommend you put it back on the shelf and get a standard sliding plate or you will always have the potential of damaging the inner dia of your new seal whenever you adjust the gearbox as someone I recall has already pointed out, unless the sliding plate is quite loose and follows the gearbox easily. However, if it's too loose, oil will seep down the back of it and out.
It will never be oil 'tight' even with standard parts but it should be vastly better than you have experienced up 'til now. However, I do understand the interest in trying for a better than standard arrangement, so it's your call.
Hope this helps
Alan