I understand that. But wouldn't it be easier to buy the 2bay offset hoppers instead and focus on something else? Up to you. I think that the old model was good enough. Of course, you can make it better (and slightly longer) than Kemtron 50 years ago.
The riveting is different on NP car.
Some of the war emergency hoppers were also around 33'. Pennsy class H31B. (1941 class H31 and post war class H31C too).
Length over strikers 32' 9"
The "generic hoppers" from Bone Valley on Shapeways is a different story. He made some very nice hoppers for his quartz sand operation in Florida. But the "generic hoppers" are not good. (Or does anyone know about a prototype suitable for those? I could not find any.) Just spending half an hour more on the design would help. Better one car correct than 4 models of nothing.