We have fitted a 500 upright litre tank to son in law's LWB van - fitted length ways so he could fit his ladders and poles and anything else long he wants to carry down each side.
You do have an issue with the fuel tank, but it's the same issue that owners of Berlingo and Dispatch vans also have. Dropping the fuel tank may seem like the best solution to put a nut on a bolt with a spreader plate, but if you need to remove the tank you have to drop the fuel tank again.
You don't say whether the tank is upright or flat.
We found that the floor just behind the back seats is an open box section which can be got at from inside the van so we were able to drill holes though that and fit spreader plates. The bolts can be reached though gaps in the support members.
If you get under the van and study the ridges of the floor you will see that the ridges are differently formed on each side between the cross member supporting the tank and the rear axel. Measure from the end of those ridges to the cross member and tranfer that to the van floor. We found we could bolt the back and front of the tank frame in securely without dropping the fuel tank.
On each side of the frame I welded lugs on and drilled holes to match up with the ridges above the fuel tank. After careful drilling these out the holes were fitted with 10mm stainless steel nutserts. High tensile bolts were used that just peaked though the other side of the floor but were far enough from the tank. (There is a space between the bottom of the floor and the top of the tank by the way.
http://www.cleanitup.co.uk/smf/index.php?topic=139067.0What's good about the early LWB Connects is they have the wiring of the people carrier that has a power point on the right hand side about where the Varistream is. Put a 15a fuse into the fuse holder behind the glove box and you have a power point. I spliced and soldered 20a wires from just behind the coupling plug and used those as a positive and negative.
Initially we tried to use this to run the pump directly from the van battery, but it kept flattening the van battery. So we later seperated the pump and put it on it's own leisure battery and fitted a split charge relay above the Varistream. As we still had a problem of the leisure battery going flat after a week of non use, we identified that the Varistream was drawing enough current on standby to flatten an 85 amp leisure battery in 2 weeks of being idle.
We have now cured the problem by fitting an isolator switch which cuts the power to the Varistream at the end of the day and prevents it draining the battery.