The only Italeri kits that I have built were the Western Stars, and it has been a lot of years since I built one of those. Like Sean said, I fully assembled the cab first. On the bottom inside of the cab are two ridges that the floor fits in between. I cut the lower ridge off, all of the way around leaving the upper ridge so that the completed interior could be inserted from the bottom with the cab fully assembled and painted. I'm sure that others probably have a better way, but if memory serves me correctly that is how I built the the ones that I built. Hope this helps.