I haven't figured out where to mount mine yet. I'd go 60" if you find the room...
Under the van seems like you're as likely to pull out a hunk of rusted metal as a jack in a year, unless you get some kind of tight storage container that fits the jack.
I know guys in the Jeep world who will take the Hi-Lift down to it's component pieces and arrange them all under the hood- a practice I never understood because when I need a Hi-Lift, the last thing I want to do is root around undoing and assembling things.
I think it needs to go somewhere you can monitor the degradation (oil and clean occasionally), somewhere easy to get to- whilst teetering on two wheels or sunk in mud up to the gunwales, and also somewhere where you won't mind putting it away after it's coated in mud and grime from getting you out of that last situation...
So if I ever figure out *where* that is on my van I'll let everybody know, but I'm really hoping someone else with a little more genius has (or will) figure it out first.