Now that we've got our pipes in place, we'll continue working on the main parts of our model.
First, we'll want to work on the exhaust ports at the back of the engine section. We'll start by just beveling things down a little:
Then, we can delete those end caps and place the cursor at the center of one of the engine openings:
At this point, you can add shapes to cut out without using a Shrinkwrap modifier—the shapes are already lined up:
When you're done, you'll need to fill in those faces. Then, we'll extrude and bevel: ...