I just finished building my first SSE (I'll try to post a couple of pictures this afternoon).  I'm using an OS 70 4s and to my amazement, balanced perfectly with the battery right behind the wingtube and under the servo tray.  I was really worried about it.  Your engine should be at least 4 ounces lighter than the OS.  I also used the same landing gear that you have chosen.  I left out the blind nuts and tapped 2 of the holes for 1/4-20 nylon screws.  I know some argue that nylon bolts are unnecessary and I finally agreed on my last plane and used stock.  That Extra 300 doesn't glide well back to the runway in a deadstick especially after a low stall turn and ripped the mounting block off 3 times when landing on rough ground. I finally changed to nylon bolts and it saved the belly just last month when that darn engine died again.

I wish I had inspected the pushrod alignment before building the plane.  The throttle cable is still tight but doesn't bind.  I didn't notice any flex in the nylon tubes, but the pushrods exiting the rear fuselage don't line up well with the control horns to achieve the recommended throws and I don't like solder links.  The pushrods were tight and sticky and didn't allow the servos to center well, then when testing the engine yesterday, one of the solderlinks disconnected.  I pulled those pushrods and replaced with solid wire using a couple of pieces of the old inner pushrod as bushings in the guide tube.  I kept the nylon clevis have some adjustment at the control horns.  I bent the rods where they exited the fuselage to line up with the control horns.  They are much smoother now.  Rather than try to trust my poor soldering abilities, I used z bends to relieve my worries.

Good luck with your, it's looking very nice so far.