I've lots of experience of this with my two collies.
Insulating the van, inside, is a good step, as is having a white van to reflect as much heat as possible.
A spinny fan fitted to the top of the van will help on windy days, or while driving to circulate the air.
On days up to 28 degrees, the van is fine (in that it doesn't get above 35 inside) above 36 degrees is the danger zone, and you really don't want your pooches in that temperature, as that's when it starts to cause damage.
In those conditions - find shade under a tree, and park there.
Also get a silver reflective windows shade for the front, the amount of heat perpetuated by the windscreen, and then soaked up by dark dash and seats is crazy. Big ones. All the way around.
Get a thermometer, and check your max temperatures often - so you have an idea of what is safe, and what is comfortable for your dogs.
For mine, if it's a hot day - they'll stay at home now. There's no point in risking it.
So, overall - if you don't need to take them on a warm, sunny day - don't.