Good work FinallyaVanner!
I'm keenly interested in getting these into production to support Vanners! Here's everything I've got which might be useful to you.
I have all the Dodge dealer's parts catalogs from 1978 to 2005, but I don't have a parts catalog for as early as a 1977. The earliest I have is my 78-81 Light Duty Truck Parts Catalog, and the section on the cargo door weatherstrip is a very short section indeed; the entire thing reads like this for all B1,2,3 models:
"Weatherstrip - Side and rear Cargo Doors B1,2,3"
4087141 - Weatherstrip, Side cargo door, Frt
4087142 - Rear Cargo door right
4081598 - Weatherstrip, Side cargo door, Rear
4087143 - Rear Cargo Door Left
3493825 - Header Side and rear w/Studs (I think this is the flattened piece you were asking about)
3729425 - w/o Studs
3498298 - Baffle, Side And Rear Cargo Doors
3759944 - Retainer, Plastic (these are the pins that hold the piece down which you were asking about)
That's all there is!
This would indicate that there definitely are differences between the side and rear cargo doors, but that the Headers were the same between the side and rear cargo doors. The weatherstrip was a constant in all B series vans, and over that entire period of time. If someone can cough up the numbers from your weatherstripping, it might just match up with some of the parts in that list.
Keep in mind that 1978 was the crossover year when many things changed... Nate might be able to get into the fine details of that.
In reference to your pictures, the side and rear cargo doors on my 1981 B250 both have the square shaped molded header, which goes on the bottom of the door. The molded header at the top is a more complicated tapered shape, with a metal reinforcing tab molded inside so the retainer pin won't rip out.
BTW, MoonShadow has a 1977 Dodge van that looks to be in really well maintained condition, it might be well worth asking what type weatherstrip is on that van compared to yours.
I have a set of new old stock side cargo door seals, plus several used sets of rear cargo door seals from various 80's & 90's vans, but they're all buried in my garage at the moment. I could probably get at them if you need more comparison info from later models.
Please keep us up to date on your progress!