John. The engine bolts to the bottom of the hardwood beams, in an inverted position. The bottom surface of the beams is actually the centerline of the thrust line, if that makes sense. I don't remember completely, but I'm pretty certain I had to modify the bulkheads holding the tank to lower it to meet the carb height. As I recall, I used part of the supplied parts, and made new pieces for the bottom to lower the tank. It was pretty simple. The way the Laser is designed (if that's the engine you're using) the carb is directly behind the intake valve, so when the engine is mounted inverted, the tank center line has to be quite low. I didn't use the supplied tank. A couple of the reviews I read said the supplied tank leaked, so I used a standard Sullivan round tank, and adjusted the bulkheads to fit. My opinion, it's going to be pretty difficult to get this exactly right without the engine while you are building, unless you have the exact measurement of the distance from the center of the engine (mounting beam) to the carb inlet. Definitely you will want the engine before mounting the cowl.