I "build" from the front rearward. Its an old habit from glow/gasoline power days, where getting the cowl and throttle linkage right was sometimes a flaming PITA, and much easier accomplished with no gear or stabilizers in place, as you wrestled about with the fuselage.
The kit comes with 4-40 engine mounting hardware. I'm sure that's perfectly adequate, but keeping in mind early reports of a recurrent tail-heavy condition, I used 6-32 bolts and blind nuts. That didn't help much, if at all, as it turns out.
However, this model does come out tail heavy… quite tail heavy, in fact. One respondent indicated he needed 50 grams of nose weight on the motor box. I believe he was using a MotroFly motor. I used the recommended E-Flite Power 32, with E-Flite's 60A Pro ESC, and I needed 110 grams (about three ounces) on the motor box to balance the model at 3" back from the LE.
The way Sig chose to design the firewall on this model is novel; to me, anyway. In fact, the parts (two main side formers, bottom former) of the motor box are, individually, rather flimsy. They all lock together when the firewall is glued in. The kit provides a small piece of triangle stock to reinforce the inside corners of the firewall/motor box area. I used much larger size triangle stock for this.
Doing so does two things; it provides for a stronger joint, and the bottom piece provides a stand-off to keep the battery away from the motor mount blind nuts. Be sure your bolts do NOT protrude past the rear surface of the blind nuts. A crash or other frontal impact could impale the end of your battery on protruding bolts…bad juju.