first, kudos on the attempts so far the videos were entertaining.

i think that the best way to deal with this is to set your goal first. if it is to jump the building, figure out how much of it you want it jump and how much you want it to fly. then you can decide what you need. from what i see and read, you won't really need a ramp. i mean do you think any (reasonable) size ramp could launch a car over a house? but you use the car to get to take off speed quickly and the ramp is purely to expedite the takeoff. so, you really need the car/plane to fly. to maintain any control over the flight, you will need to have at least control of the attitude of the plane. lack of it leads to disaster, like your last jump in the video. this means that the car/plane will need to have elevator control unless you have a canard. adverse rolling can be solved by higher dihedral. yaw control is among the least important IMHO.

so when you make the frame, you may want to consider putting an elevator somewhere.

as for tapping the power from the car, it's not easy since the engine is transversally mounted. if you do go for a second powerplant on the plane itself, i'd recommend that on the maiden jump/flight, you don't jump it. see if you can take off on the prop alone... this will help to prevent total destruction from the first flight!!!