IF not covered, does it have sheet balsa tail surfaces, or built up? If sheet balsa, you could cut (Drill) some holes in the surfaces. Or try lighter pushrods (as CafeenMan suggested).

Other than that, it sounds as though you've either done, or already suggested, other options. (Spinner, engine forward, heavy batt pack.) A Higley "Heavy Hub" prop nut would give you an ounce or two more on the nose, but spinners don't work with them. At least they haven't for me.

As for moving the engine forward, you can put a 1/4 inch plywood plate between the mount and the firewall. Pretty easy to do.
Dennis-