Just doing some reading over at EAS, and noticed that the Lancelot and Sirius use the same saucer as per the Sirius' notes. Just thinking that you can probably solve your issue by following the Sirius' example and place the impulse above the saucer as per the Sirius noting that that makes the Lancelot's image on EAS incorrect.
Another idea I got on the other hand viewing the schematics is an saucer underslung impulse structure blocked by the front view, but is seen on the side view. Both are more in keeping with the typical Starfleet designs we usually see during this period and later.. usually the impulse engines are on the saucer with the warp engines being on the secondary hull. That way your saucer can move away if necessary under impulse if you have to boogie away from your overly damaged and ready to explode warp core. I'll see what I can come up with for you based off your images...