Jim's advise is good. Your idea of weight on the front of the motor mounts is also a good idea. weight on the firewal is not as effective in balancing, so you will require more weight.

I have built and flown two of the 4*60's and both were tail heavy. The second wasn't quite as bad. I used the stock motor mounts with 5/8"sq x the height of the mount of steel and bolted the mount to the firewall using this as a spacer. This was about 6 oz of steel. On the second, I used the Dubro vibration mounts and this moved the motor even futher forward than the spacers. I still needed a couple oz more and Imade a bar of 1/8 thick lead about1" wide shapped in a U with tabs on bot h of the uppers that I drilled for the motor screws. I thread my motor mounts rather than use nuts, so I just used a longer screw on the front two and then adde a flat washer and a nut with thread lock. That and moving the battery as far forward as possible put it in good balance.

Don