I have built many PT-40's and the PT-60> I have found that the PT_60 does have problems with cutting and fitting properly. as to your question, yes fill the gap with wood (ply) and use Epoxy as moosimo1 said. JUST MAKE SURE that you get the firewall properly lined up. I also find it to be a very heavy plane. it seems to be also very tail heavy. I am in the process of cutting lightning holes in the aft section of the fuse to remedy this problem. I am using a OS 70 four stroke also. If you havent built the wing yet build it with the sport wing version (you have to cut the spar joiner) you will get more fun out of it. I use this plane to help other new pilots learn to fly. its very durable and easy to repair. have fun!