VGN64,
The Lionel car is a model of the PRR X31. The Seaboard car is different. The roof, sides, side sills and ends are different. But the Lionel car is a 40 foot car with a round roof, so if the details are ok with you then go for it.
Smoky Mountain Model Works has said he wants to do the cars as a resin kit but he has released a flat car and is taking orders for a gondola and still no boxcar yet.
http://www.smokymountainmodelworks.com