Having a separate hood, walkway, and cab is the best way to go, IMO. It's easier to paint that way. Using stock walkways and cabs allows you to use stock walkways and window glass. Here's a GP15T I designed a few years ago. Only the long hood needed replacing. I added the bumps on the inside of the shell so that it snaps into place just like the original Atlas hood.

BTW, I noticed that the resin shell has a correct square roof section behind the cab but the 3D shell doesn't. The 3D shell doesn't have any walkway tread, either.