It'd probably be easy done if the body was off the van. The VS sedan tank is long and flat, HZ commercial is sort of square - very different shape. Not sure there is enough room on a ute/van for it either in the original position. I tried fitting one in my Overlander tonner and it just fitted between the cab member and the shocky member and it'd fit lengthwise but that is where ute/van chassis is shorter. Where it would fit is where the spare wheel is, simply strap it against the steel that the spare pushes against. Not sure how you'd go for a filler though. I do have a circuit you can build to make a VN tank sender operate a HZ fuel gauge if you go that way.
Just thinking it might fit up above the line of the rails against the body above the original tank's position, but it'd mean strapping it to the floor and hence holes through the floor. Might also be too high for a filler.