I'm a loyal Dodge guy, but they had a rear weight on the PUs too.
To the OP, these vans were designed before the initial safety push in the mid-60s. For you to retrofit safety into the design, you will have to incorporate a lot of changes that will by necessity change the character of them and as noted, make changes to the overall design to bring back some semblance of road manners.
I can suggest the following not as something I have done, but as levels that would seem to make sense.
Better braking and handling. These have been done and will make it more modern and increase your ability to avoid collisions.
Incorporate stronger seat mounts and 3 point belts. This is the first step in keeping you in place.
Add structure to the nose, tied into the hinge area and add door beams, to pass energy around you to the body.
Move the driving position back some. This would require moving the seat back, tilting the steering column back and moving the pedals back. Side vision will be affected. Dodge did this in the 94-up B vans compared to earlier B vans.
Install a roll cage.
Move the engine/trans back and use an IRS so the driveshaft can be eliminated. This will allow the doghouse to move back and allow you to move the seating and controls back also.
Install active belts and air bags.
Most of this is going to be expensive and require a lot of thought, engineering and time.
It would be easier to move to a different vehicle than to really make one of these 'safe'.
You could go to a later American van for a slight increase, a later japanese van, or a japanese van with a conversion if you must have the look (sorta).