I know this is your first large gas model so a K&B GH .19 is probably not a bad choice, although as Steve pointed out that design was for the K&B 3.5. On my Witchawk 500 that I flew at the Nats the requirement for a nostalgia engine meant I used an OS 19, which is quite a bit lighter than the K&B 3.5. With a backplate mount, I had to move the pylon quite far back, probably an inch and a half from the firewall. This made the model somewhat squirrely in the climb and made having the right warps super critical. Ideally, the pylon should be right up against the firewall, so I would use an aluminum mount with your Greenhead .19 to get some weight up front, so that when you move the pylon forward or back to get the balance point it ends up as close to the firewall as possible.