We have also tack welded together our rear back plane and the rear engine members. Our engine members were then tacked to the back of our bottom plane.
After the engine members and the rear back plane had been tack welded, there was some concern as to whether the engine and the gearbox would be able to fit into the engine area. As a result, we took an old engine from a previous car and made a slightly oversized mockup of the gearbox to see if they would fit inside the engine area. Both fit well within the back area of the car.