My ears are burning on firewall placement.
It really does not matter so much on the twins I have bashed. What is important is that you keep weight down. My trick is to build the nacelles less the firewall and build them too long. Install the engines and mufflers on the firewalls and move them both forward and back until the plane balances. Mount the engines in this place. This prevents having to add a bunch of additional balance weights.
My two cents on the landing gear.....if this if your first twin.....keep it a tricycle gear. The engines never come up together and a tail dragger is harder to handle with differential thrust.
Hope this helps. Good Luck,
Twinman
PS You do have to send pictures for the rest of us.