Since you've pinned the firewall to the fuse sides it's probably not necessary, but I'd still consider epoxying a couple pieces of 1/4-3/8" triange stock between the back of the firewall and the fuse side. It might be overkill, but it's additional safety for minimum weight gain.

Are you going to put foam around the tank? If so, make sure the hole in the former is big enough so you can stuff it through, or glue the foam in place before you put the top/bottom on the fuselage. Will you cut notches in the bottom of the fuse for the landing gear block?