Seth mate, that sur file is horrible.
You add on roughly 50% extra to the ship's height with it.
Do the sur splicing tutorial that Gurjiv put up. It shouldn't take longer than an hour or two and you can have a perfect sur file for it instead of..this.
If you don't want to do that, a simple cube sur file is better than the currrent one, all it would add would be a small amount to the engines at the bottom and the gap between the hatchet at the front.
It actually makes no sense to have the highlighted area, ought to just have a flat bottom on the sur instead.
At least squish the highlighted area a bit plox.