As you may know:
1. The driver side door has latches/catches at top and bottom. This is the foundation of the two doors. You must be sure this door is not moving.
2. The passenger door is only held by the driver door, in the middle, at the latch you mention.
3. Once the driver door is tight at top and bottom, the passenger door catch can be adjusted in and out to obtain tight closure.
4. I have had many 55-59 suburbans and panels. The barn doors were not always 100% tight.
5. If you use proper, quality rubber on both doors, this will allow squeeze for anti rattle, weather seal and a semi tight closure.
6. The handle, latch, striker are same as front door.
7. These doors are prone to being bent by all manner of abuse. Which would aggravate closure fit. The hinges are prone to be loose, bent, rusted, sagging, need adjustment. This will cause your trouble to be multiplied.
8. Slamming the passenger door hard and still loose is curious. Sounds like you are not compensating for rubber rebound or have the wrong rubber or wrong installation. You will need new Steele Rubber Co. rubber for this to ever work like you want, provided ALL proper mechanical parts are present and doors and sills are in good condition.
9. You will need to carefully examine the situation to discover what exactly is loose, where and why. Be mindful that either door, or both, can be at fault together with the other items I've mentioned. Check the actual opening for previous accident damage/repair, etc.
10. I don't know what your "loose" classification is for sure. Loud rattling would be annoying and leaking would not be good.
I learned something here. I did not know there was any dust in Alberta.
Are you using a passenger door striker in the rear?
