You'd need to check the loading limits but I reckon if you had the 650 litre tank mounted immediately behind the second row of seats, you may be within your axle limits. Yes, we do have custom tanks made for clients. The great advantage of them is that it allows you to have a tank that fits the exact space and can be made to hold buckets, pumps and various other components. The downside is they are more expensive, so if you can easily fit a standard tank in, then it's worth doing.
Have you thought about a VW Transporter as they have a galvanised body which is ideal for our profession.