Sorry cant help with the Engineering fabs up that way .
Not being rude, but it seems it would be a lot of effort, expence and weight added to slot an upgraded chassis under the van, you say whats already there is sound, Would it not be more beneficial just extend the drawbar or just the hitch area and fit your desired suspension axle upgrade to the existing chassis, strengthening / modding where needed to accept a bolt in / weld in suspension cradle package .
Got to remember this van would have 44 year old timber frames, rusty old screws, dried out age affected glues [ if any ] and staples holding things together inside its walls / roof, Would think I'd be more concerned about the body holding up for any Distance travel or offroad work it might see .
Could you post up some pics ? ..might help with being able to give some advice on how to go about the upgrades .