Bobt, I haven't seen any 3point seat belts in an Atr deco truck. I have lap belts in mine, but you might be able to weld in some sort of bracket as suggested. The back wall isn't very thick so some reinforcement will be necessary.
Another option if to find some bucket seats with the 3 point seat belt built in. I think a few buicks had them. I'm not sure about the width of these seats though.
Let us know how it comes out. What ever you do.
George
