Those particular cars appear to be an American Car & Foundry product (interestingly with Pullman Standard style ends) and differ in a lot of details from my FMC car. Montana Rail Link did have 10000 series Plate F cars built by Gunderson that were very similar in design that I could be a fairly strait forward conversion of my SP car.
I can't describe how excited I am for the SP double doors! Your boxcars are among some very select rolling stock that actually pulls like they weigh a realistic amount.