I would not be too worried about a bit of lead in an LT-40.

My LT-40 balanced with a bit of lead in behind the firewall, a 1000 mAh battery under the tank, and the receiver just behind the tank. The engine is an old OS Surpass 48. Not a big heavy engine.

I balanced it just ahead of the most rearward CG recommended in Sig's manual. It is still plenty stable. Even at the CG, it still won't spin properly (i.e. its pretty stable), and I will be nudging it back further.

Total wet weight is between 6.5 and 7 lb. But then, I covered it with koverall and plenty of dope...which is on the heavy side. Still it floats. That's a wing loading of only 18 oz/sqft.

The tail wheel, and gear, will have a much larger affect than the push rod arrangement because they are both heavy and hang off the end of the tail. If you are concerned about balance get the lightest tail hear possible, and a light tail wheel. Mine is a big heavy CBA/Tatone leaf spring tailgear, with Dubro tail wheel. Neither are lightweights. doggscube's setup look's pretty light.... at least lighter than mine.