Ed,
I'm curious to see how much common tooling there really is. The two cars are different and represent 15+ years of freight car development between them.
I built a couple of GLa's out of the IM USRA twins and they are different. Besides the rib contours being different, the cars have 6" difference in inside length, 8 inches of difference in top of side sill height and 4 inches of difference in bottom of side to top of rail height. I could never correct the 4 inch difference in top of rail to bottom of side height because it meant pushing the under frame up into the car more.
I know that doesn't sound like much but it really is visible.
We'll see, but I hope Lionel does the GLa right and the USRA Twin a little off. O Scale already has a really beautiful USRA Twin, it could use a good plastic GLa. Worst case is if they try to find a happy spot in between both and do them both wrong.
George Losse