The WP welded flats altered for wallboard service, starting in 1955, were 56' PS4s and then Thrall cars of the same design. Other riveted cars of shorter designs were on the roads roster as well, from a variety of builders including PC&F and ACF. However, I don't think the WP ever owned GSC flats.
The UP still has some of the ex-WP welded cars in revenue and MOW service. I don't know that they ever parted with any of them to any other road, though I've seen some very similar flats caught on the NWP's severed north line around Eureka. I never verified whether these were ex-WP or SP cars or of another heritage.
I'd love to see photos of the NSN models though. I suggested these cars recently to N Scale Kits and they asked for more info, so I'm keeping my fingers crossed they'll be available in the near future.