Hi
Need to add to the above.
Currently Ford have 2 crew vans, in either a short or long wheel base version, and both have lower payloads than their van equivalents.
The LWB crew van has a payload of 755kgs as opposed to 888kgs of the T230 LWB.
(The short wheel base has a payload of 718kgs, as opposed to the T200 with 634kgs and the T220 of 829kgs.)
Looking at the specs of the LWB crew van and T230 LWB van they are similarily plated, so the reduced payload is because of the additional equipment carried, ie seating etc.
I would contact Ford Technical and ask them to tell you what weight the rear axle will carry if the weight is to be put behind the rear seats and that should give you a starting point. Also you may try your local weigh bridge as they should be able to tell you what weight is on the back axle with the vehicle empty, and this amount subtracted from the max axle weight will be the figure to work from.
IMO, with the tank that far back, I would ensure that the fitting is done very securely. That tank could have a long way to fly in the even of an accident.
Spruce