I think you will struggle to fit 300mm discs inside 14" wheels. You did not mention what front axle your van has, the LWB type with the wide offset wheels, or the SWB style with the flat disc wheels? I know you guys get the single wheel vans in OZ with 6 stud wheels, just to add to the confusion!
FWIW, I have found a combination that I am going to use on my van, (it is a LWB Mk1 with the dual wheel style front wheels) I have bought a pair of "black diamond" front vented discs from the UK, 270x24mm, and am fitting a pair of LDV Convoy 4 pot front calipers to this. They look similar to the Transit ones, but have larger pistons (46mm vs. 40mm for the Transit) and about 25% more pad area, and are physically larger . The best part is, they just happen to bolt straight up to the Transit caliper mount, without ANY mods! Mintex do the M1166 race style pad to suit these calipers, so I have a pair of those on the way too. I am fitting a Territory 4.0 Turbo engine to mine, so need all the stopping power I can get
Assuming you have the 6 stud LWB type wheels, you will have to find a pair of these to attach your discs to:
and you will also need the stub axles from a disc braked van, as these are different. The stub axles can be from a SWB single rear wheel van or a LWB with dual rear wheel type van, they are both the same, either Mk1 or Mk2.
This is what the vented discs and Transit calipers look like:
I will put some pics up in the next month or so of the brakes on my build thread, so keep an eye here: viewtopic.php?f=13&t=117844&start=15