Well, I finally have both doors on my 1950 Ford coupe closing so they pretty well match the adjacent body panels. Thanks again to all of you who sent along your suggestions for solving my problem. I ended up using yet another solution which I'll do my best to explain right now.
Turns out that both of my doors fit pretty well in their respective apertures, and also fit pretty well in terms of curvature of the panels. But they would never close snugly and easily for me, no doubt due to the fact that I never completely closed the doors after mounting the aftermarket door rubbers, which would have squeezed these seals properly so they could take a less-thick "set".
Here's what I finally did. I put oversize washers behind the 1-1/2 inch holes located inboard from each door lock assembly, attaching each of these to a pair of old 2-inch cylinder head capscrews, washers and nuts so that the capscrews' threaded ends faced toward the center of my car. Then I attached two stout chain lengths so that these in turn could ALSO be bolted to one another at the center of my car. Next, I placed a small screw-handle tire jack on the center of the driveshaft hump sheet metal between my two aftermarket seats. Working through one of the door window apertures, I placed scrap 2x4 blocks over and under the jack until I could expand the jack easily by cranking its handle. Then it was just a matter of cranking until I could hear the wood popping and crackling, giving me an idea of how much pressure I was applying evenly to my doors. Once i had backed the upper half of my body out of the window cavity, I saw I had obtained a pair of doors that closed tightly and evenly against the body, by putting even inward pressure to further collapse the two rubber door seals. After that, I simply left things in place for a solid week before removing my device to find how nice the car looked.
I know. Those of you with solid front seats are probably going to have to remove the seat temporarily to make this idea work, or else discover some other solution which you can post here on the Forum so others can use it. Anyway, I hope these gyrations of mine help some of you, and best of luck!