Make sure you use a big, wide strap (2" one not one of those nancy boy 1" toys) and the front of the rack is really well secured into the ute.
Mine used to be secured by 4 turnbuckles. @ at the transom, 2 @ the front seats.
How long is the tray??
If I remember Ok they're about 6 feet or 1.8m. So if the tinny is 3.5 you could put .6 (I think that is still OK but I'm sure I'll be informed if not allowed anymore) over the back, then 1.8m over the tray and that'll leave 1.1m over the cab. That isn't too much I don't think but hopefully others will have an opinion.
IMO it would be quite OK to have the front strap at about the same place as the front seat. That's a strong point for tying down and the boat itself should be strong enough the have the front bit hanging without support.
Just make sure the tinny will not jack-knive into the van though when your backing.
How much movement is there between the cab and tray??
Sounds like a holiday happening..good on ya's