I suspect that if you look closely at the existing weatherstrips, you might see that they were made out of pieces bonded together rather than being one big, molded weatherstrip. Of course, the item at the bottom of the A-pillar/upper cowl would be its own molded item.
If the "plug" is still sealing decently well, but is cracked, sometime I've had good luck with the heavier-bodied engine sealant to fill them and then to skim-coat the rest of the rubber with the sealer, filling in minor cracks in the process. This is not an extremely long-term fix, but can work in the interim. I use the BLACK sealer so it matches the rubber and also puts a bit more "black" into the weatherstrip itself, making it look better in the process.
ONE thing about NOS weatherstrips. They are usually as old as the car is, so they might be new, but not "new" as if they were produced recently. They might feel soft, similar to a recently-produced item, but as they are usually stored in a box, a bit wadded-up, it might take a while for them to gently "unwind" from that storage, at best. Result is that they might be new, but it might well be that some kinks will still remain in the rubber. So they will need to be treated gently. Not to forget there might be a little bit of shrinkage, too.
Back in the later 1970s, the lh frt door on our '72 Newport Royal 4-dr sedan never had sealed really well, so I ordered a new door weatherstrip from the local Chrysler dealer. When it came in, it was packaged in a 6'x7'x3" cardboard box. Most of the bends and twists (to get it into the box) in the rubber were a bit gentle, but not all of them were. When I pulled it out, it did not lay flat. I put it back in the box and archived it for later consideration, where it still is.
Steele is a premier weatherstrip provider, but will offer "roll goods" rather than "made to order" items (for which there is a demonstrated long-term demand for the model-specific weatherstrips.
You might also need to emprovise a bit. For example, there are similarities to "sedan" front doors and "hardtop" front doors. So you might adapt a 2-dr hardtop weatherstrip to your 4-dr sedan front door. Carefully snipping the top and rr top of the hardtop weatherstrip and mating it your sedan weatherstrip.
More people restore 2-dr hardtops and convertibles than 4-drs, as a general rule, but sedans' weatherstrips deteriorate too. Perhaps others have some other information or links to possible suppliers?
Hope this might help,
CBODY67