275/75/16 would be the perfect size for most D40's but only BFG and Maxxis make tyres in that size in Australia, I believe coppers have them in the USA, but MT's don't do it at all.
In NSW you can fit any size up to a 33" with approval and inspection of an engineer. No engineer will pass anything bigger any more. As for insurance, they can not reject anything approved by the RTA. They may try to put it over you but just put them in there place.
Trust me , i know this , god could design something for you truck , and if if wasn't in the financial interest of the engineer , it wont gat passed , most derive their income from insurance or big business , good luck with that