Quoting Andrew AKA Hump-Free
Hey Man Thanks For Joining HFR!!! So, the issue you're having is the lack of interior room, I assuming that it is in regards to fitting MiniFigs. In order to achieve this, without adding height, is rather difficult but you can do it depending on the vehicle. In this case, you can add some interior room by building downwards, meaning you keep the exterior mostly the same as you modified the underbelly of the vehicle, in order to fit the MiniFigs. From what I am able to see, that is the best course of action.
Thanks. I am happy to be part of the group.