I would seriously advise not to put a 650l tank in a connect 900kg payload the lower payload one would be total madness and bad of anyone wanting to supply you said item, is not enought for a 650l system, it is not tight it is to small a payload, add frame weight tank weight, hose reel, fuel, you, other bits you are over loaded, 2 in one van and you are going to be way over,
if you have not bought the van get think again and get a bigger van, or get a smaller system.