I did this a good few years back, but I seem to remember the straps are fixed at one end in the tank compartment. The other end has a bolt on it which goes through the floor. Release the nut on the other end and the tank will come out.
Under the van look at the crossmember inside the crossmember there will be a nut on a long thread They will be rusty so a good spray with plus gas will be needed along with a deep socket
defo do this. loads of plus gas and the leave it a bit to penetrate. get someone to hold the straps still while you turn the nuts or they may spin and tear. As said they will be rusted up so be patient. when you put them back clean the threads with a tap. use new bolts and copper grease.
It's crazy that the strap ends are only held on by a couple of small spot welds. I nearly broke mine.
Im almost at that point. I gave up tonight trying to loosen off the bolts. Rusted to bugger so soaked them and will try in the morning.
Managed to get them off and remove the tank, but.... i snapped a strap! Also stripped the thread. Got enough to do so will worry about replacing it later. Nice and clean petrol tank area now though!