ORIGINAL: speedracerntrixie
The best thing to do and the most difficult is move anything forward that you can and then remove weight from the tail. You would be surprised what can be done here. On my last project that came out tail heavy, I replaced all the steel hardware in the tail with aluminum hardware. I even made an aluminum wheel collar for the tail wheel. That and moving my RX battery, moving the motor 1/4'' forward fixed the CG and the airplane gained no weight.
I would agree with what others said, that 6oz isn't huge. Probably small enough that I would mount the weight to the bottom of the engine mount, with zip ties or something similar and go fly. If you like the way it flies, then make the mounting permanent. It is likely you are going to want to move the CG some any to fit your style of flying and comfort zone anyway, which will involve adding or removing nose weight. Once you are comfortable with how it flies, you can decide if you want to try and remove weight from the tail.
I don't know the ratio of fuselage length in front and behind the CG on the Sig Somethin Extra, but on the last plane I was working with (an Edge 540), it was about 1:3. One inch in front of the CG to three inches behind. A tape measure or yardstick will let you calculate it for your plane. It doesn't have to be to the nearest 16th of a inch. An approximation is good enough. Here is why I point this out. This ratio controls the ratio of weight you will have to add to the nose, or remove from the tail. It is the inverse. For every ounce you removed from the very end of the tail, you could remove three ounces from the tip of the nose and keep the same balance point. So, measure from front where you mount the extra weight back to the CG and from the CG back to where you can remove weight, by changing the tail wheel hardware as an example someone gave. So, if you can save a half an ounce in the tail by putting light tail wheel hardware in place and the ratio from front weight to CG and CG to tail wheel is 3:1, the that half an ounce saves you 1.5oz up front.
This is also why moving the engine forward likely won't have a lot of impact. The math here is greatly simplified for illustration and all the weight is treated as being at a single point. Assume your fuselage is 48" tip to tail and 1:3 ratio in front to behind the CG. Also assume that you are having to add 6oz of lead to the very front to balance and you have an engine that weighs 21oz. You have 27oz in front of the CG balanced by 9oz behind. Now, shift the engine forward by an inch. This changes the ratio to about 1:2.75. You still have that 9oz in the tail that now needs just about 24.9oz in the nose to counter it. Subtract your 21oz engine and you still need almost four ounces to balance. Move it forward another inch and you will still need two ounces to balance and now you have an engine that is two inches further forward than originally designed. That is not a trivial amount.
It is up to you to decide if it is worth the time, effort, and expense to shift weight out of the tail. Hence my suggestion of "balance it with weight in the nose and go fly". If you decide that it is ok with the extra weight, you can also look at getting something useful out of the weight. A bigger battery pack for more flight time, or a slightly larger engine for more power for example.