How Can I Cut a 4 Door Pickup into a 2 Door Cab

Hi. I have recently seen Yard's awesome new ambulance, and have been wondering. Someone in the comments in the gallery recommended a Bravado Bison ambulance. So, I found an unlocked Bison, which is a 4 door pickup. I know how to cut certain parts off, but my problem is making it look attached to the rear part of the ambulance. How would I add a backboard to the cab (just behind the driver/passenger seats)? Then would I just make a slim rectangle to connect the cab to the rear end (emergency medical part thing)?

Thanks.

(The reason I would like to do this myself, and not have someone else make the ambulance for me is because I would like the knowledge for later, +I would like the customization in terms of lights, liveries, etc. I also understand Yard's ambulance is unlocked, however, I don't think the connection from the cab would work with the Bison's cab.)