I'm not understanding you said you had to trim it a lot to keep it leveled up (nose heavy) then you said it was tail heavy with the cowl off?... You must have had some other problem, if your plane went mad with that cowling off.... it is a light cowling and should not throw the cg off by that much. (or your CG was slightly to the rear to begin with).
If you do a 45* up-line and go inverted your plane should nose over ever so slightly, if it wants to push toward the sky your cg is aft and if it keeps a strait line you have a more neutral CG. (also it should fly strait and level when upright before doing this test).